Apparatus and method for horizontal drilling

ABSTRACT

An apparatus and method for horizontally drilling provides for detecting subsurface features and avoiding such features during closed-loop control of an underground drilling machine. A horizontal drilling system includes a base machine capable of propelling a drill pipe rotationally and longitudinally underground. A cutting tool system is coupled to the drill pipe, and a control system controls the base machine. A detector is employed to detect a subsurface feature. A communication link is utilized for transferring data between the detector and the control system. The control system uses the data generated by the detector to modify control of the base machine in response to detection of the subsurface feature.

This is a divisional of U.S. patent application Ser. No. 10/224,205filed on Aug. 20, 2002, to issue on May 3, 2005 as U.S. Pat. No.6,886,644; which is a divisional of Ser. No. 09/676,730, filed on Sep.29, 2000 and issued as U.S. Pat. No. 6,435,286; which is a divisional ofSer. No. 09/311,085, filed on May 13,1999 and issued as U.S. Pat. No.6,161,630; which is a continuation of Ser. No. 08/784,061, filed on Jan.17, 1997 and issued as U.S. Pat. No. 5,904,210; which is acontinuation-in-part of Ser. No. 08/587,832, filed on Jan. 11,1996 andissued as U.S. Pat. No. 5,720,354, which are hereby incorporated byreference herein in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of trenchlessunderground boring and, more particularly, to a system and method forhorizontal drilling and subsurface object detection.

Utility lines for water, electricity, gas, telephone and cabletelevision are often run underground for reasons of safety andaesthetics. In many situations, the underground utilities can be buriedin a trench which is then back-filled. Although useful in areas of newconstruction, the burial of utilities in a trench has certaindisadvantages. In areas supporting existing construction, a trench cancause serious disturbance to structures or roadways. Further, there is ahigh probability that digging a trench may damage previously buriedutilities, and that structures or roadways disturbed by digging thetrench are rarely restored to their original condition. Also, an opentrench poses a danger of injury to workers and passersby.

The general technique of boring a horizontal underground hole hasrecently been developed in order to overcome the disadvantages describedabove, as well as others unaddressed when employing conventionaltrenching techniques. In accordance with such a general horizontalboring technique, also known as microtunnelling or trenchlessunderground boring, a boring system is situated on the ground surfaceand drills a hole into the ground at an oblique angle with respect tothe ground surface. Water is typically flowed through the drill string,over the boring tool, and back up the borehole in order to removecuttings and dirt. After the boring tool reaches a desired depth, thetool is then directed along a substantially horizontal path to create ahorizontal borehole. After the desired length of borehole has beenobtained, the tool is then directed upwards to break through to thesurface. A reamer is then attached to the drill string which is pulledback through the borehole, thus reaming out the borehole to a largerdiameter. It is common to attach a utility line or other conduit to thereaming tool so that it is dragged through the borehole along with thereamer.

In order to provide for the location of a boring tool while underground,a conventional approach involves the incorporation of an active beacon,typically in the form of a radio transmitter, disposed within the boringtool. A receiver is typically placed on the ground surface and used todetermine the position of the tool through a conventional radiodirection finding technique. However, since there is no synchronizationbetween the beacon and the detector, the depth of the tool cannot bemeasured directly, and the position measurement of the boring tool isrestricted to a two dimensional surface plane. Moreover, conventionalradio direction finding techniques have limited accuracy in determiningthe position of the boring tool. These limitations can have severeconsequences when boring a trenchless underground hole in an area whichcontains several existing underground utilities or other natural orman-made hazards, in which case the location of the boring tool must beprecisely determined in order to avoid accidentally disturbing ordamaging the utilities.

Recently the use of ground penetrating radar (GPR) for performingsurveys along trenchless boring routes has been proposed.Ground-penetrating-radar is a sensitive technique for detecting evensmall changes in the subsurface dielectric constant. Consequently, theimages generated by GPR systems contain a great amount of detail, muchof it either unwanted or unnecessary for the task at hand. A majordifficulty, therefore, in using GPR for locating a boring tool concernsthe present inability in the art to correctly distinguish the boringtool signal from all of the signals generated by the other features,such signals collectively being referred to as clutter. Moreover,depending on the depth of the boring tool and the propagationcharacteristics of the intervening ground medium, the signal from theboring tool can be extremely weak relative to the clutter signal.Consequently, the boring tool signal may either be misinterpreted orundetectable.

It would be desirable to employ an apparatus for detecting a natural orman-made subsurface feature and controlling an underground excavator toavoid such subsurface feature with greater response time and accuracythan is currently attainable given the present state of the technology.

SUMMARY OF THE INVENTION

The present invention is directed to a system and method of horizontallydrilling and subsurface feature detection. According to one embodiment,a horizontal drilling system includes a base machine capable ofpropelling a drill pipe rotationally and longitudinally underground. Acutting tool system is coupled to the drill pipe, and a control systemcontrols the base machine. A detector is employed to detect a subsurfacefeature. A communication link is utilized for transferring data betweenthe detector and the control system. The control system uses the datagenerated by the detector to modify control of the base machine inresponse to detection of the subsurface feature.

The subsurface feature may be a geological or man-made obstruction, inwhich case the control system uses the data generated by the detector tomodify control of the base machine to avoid contact between the cuttingtool system and the obstruction. The subsurface feature may alsocomprise a transition between a first subsurface geology and a secondsubsurface geology, in which case the control system uses the datagenerated by the detector to modify control of the base machine tomodify one or both of cutting tool system direction and base machinepropulsion in response to the detected subsurface geology transition.Cutting tool system and/or subsurface feature location and depth may becomputed.

The detector can be integral with the cutting tool system. In such aconfiguration, the cutting tool includes a cutting element, a powersource, a transmitter, and a receiver. In another configuration, thedetector is communicatively coupled to the cutting tool system. In afurther configuration, the detector operates cooperatively with thecutting tool system to detect the subsurface feature. In yet anotherconfiguration, the detector is situated above ground. According toanother configuration, elements of the detector are respectivelysituated at or proximate the cutting tool system and above ground. Thedetector can include a ground penetrating radar unit, a beacon or anacoustic wave detection unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a trenchless underground boring apparatus inaccordance with an embodiment of the present invention;

FIG. 2 is a detailed schematic side view of the trenchless undergroundboring tool and a probe and detection unit shown in FIG. 1;

FIG. 3 is a graph depicting time domain signature signal generation;

FIG. 4 is a graph depicting frequency domain signature signalgeneration;

FIGS. 5 a-5 c show three embodiments for passive microwave signaturesignal generation;

FIGS. 6 a-6 d show four embodiments for active microwave signaturesignal generation;

FIGS. 7 a-7 b show two embodiments for active acoustic signature signalgeneration;

FIG. 8 shows an embodiment of a cooperative target incorporating asignature signal generator and an orientation detector;

FIG. 9 is an illustration of an orientation detector for detecting anorientation of a cooperative target;

FIG. 10 is a block diagram of an orientation detector which, inaccordance with one embodiment, detects an orientation of a cooperativetarget and produces an output indicative of such orientation, and, inaccordance with another embodiment, produces an output signature signalthat indicates both a location and an orientation of the undergroundboring tool;

FIGS. 11 a-11 b illustrate an embodiment of an orientation detectingapparatus which includes a number of passive signature signal generatingdevices that provide both boring tool location and orientationinformation;

FIG. 12 illustrates another embodiment of an orientation detector thatproduces an output indicative of an orientation of the undergroundboring tool;

FIGS. 13 a-13 b illustrate another embodiment of a passive orientationdetector that produces a signature signal indicative of both thelocation and orientation of the underground boring tool;

FIG. 14 illustrates an embodiment of an orientation detector suitablefor incorporation in an underground boring tool that produces an outputindicative of the rotational orientation and pitch of the boring tool;

FIG. 15 shows an embodiment of a boring tool incorporating an activesignature signal generator and an orientation detection apparatus;

FIG. 16 is a diagram of a methodology for determining the depth of anunderground boring tool incorporating a cooperative target by use of atleast two receive antennas and a single transmit antenna;

FIG. 17 a is a depiction of an underground boring tool trackingmethodology using an array of two receive antennas and a transmitantenna provided within the receive antenna array;

FIG. 17 b is a graph illustrating signature signal detection by each ofthe antennae in the receive antenna array of FIG. 17 a which, in turn,is used to determine a location and deviation of an underground boringtool relative to a predetermined above-ground path;

FIG. 18 a is a depiction of an underground boring tool trackingmethodology using an array of four receive antennas and a transmitantenna provided within the receive antenna array;

FIG. 18 b is a graph illustrating signature signal detection by each ofthe four antennae in the receive antenna array of FIG. 18 a which, inturn, is used to determine a location and deviation of an undergroundboring tool relative to a predetermined above-ground path;

FIG. 19 is an illustration of a single-axis antenna system typicallyused with a ground penetrating radar system for providingtwo-dimensional subsurface geologic imaging;

FIG. 20 is an illustration of an antenna system including a plurality ofantennae oriented in an orthogonal relationship for use with a groundpenetrating radar system to provide three-dimensional subsurfacegeologic imaging in accordance with one embodiment of the invention;

FIG. 21 illustrates an embodiment of a trenchless underground boringtool incorporating various sensors, and further depicts sensor signalinformation;

FIG. 22 illustrates an embodiment of a trenchless underground boringtool incorporating an active beacon and various sensors, and furtherdepicts sensor signal information;

FIG. 23 is an illustration of a boring site having a heterogeneoussubsurface geology;

FIG. 24 is a system block diagram of a trenchless boring system controlunit incorporating position indicators, a geographical recording system,various databases, and a geological data acquisition unit;

FIG. 25 is an illustration of a boring site and a trenchless boringsystem incorporating position location devices;

FIG. 26 illustrates in flow diagram form generalized method steps forperforming a pre-bore survey;

FIG. 27 is a system block diagram of a trenchless underground boringsystem control unit for controlling the boring operation; and

FIGS. 28-29 illustrate in flow diagram form generalized method steps forperforming a trenchless boring operation.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring now to the figures and, more particularly, to FIG. 1, there isillustrated an embodiment of a trenchless underground boring systemincorporating elements for controlling horizontal drilling andsubsurface feature detection. In one embodiment, the detection systemincludes an above-ground probing and detection unit 28 (PDU) and abelow-ground cooperative target 20 mounted to, contained in, orotherwise coupled to an underground boring tool 24.

The PDU 28 and the target 20 operate in cooperation to provide reliableand accurate locating of an underground boring tool 24. In addition, theorientation of the boring tool 24 during operation may also be provided.In terms of general operation, the PDU 28 transmits a probe signal 36into the ground 10 and detects return signals reflected from the groundmedium and the underground boring tool 24. The return signals typicallyincludes content from many different reflection sources, often renderingdetection of the underground boring tool 24 unreliable or impossibleusing conventional techniques. Detecting an underground boring tool 24is greatly enhanced by use of the cooperative target 20, which, inresponse to the probe signal 36, emits a signature signal that isreadily distinguishable from the return signals reflected by the groundmedium and the underground boring tool 24. The cooperative target 20 mayalso include an orientation detection apparatus that senses anorientation of the boring tool 24. Boring tool orientation informationmay be transmitted with the location information as a compositesignature signal or as an information signal separate from the signaturesignal. As such, the presence, location, and orientation of anunderground boring tool 24 is readily and reliably determined byemploying the probing and detection system and method of the presentinvention.

It is well known in the field of subsurface imaging that conventionalunderground imaging techniques, such as those that employ GPR, detectthe presence of many types of underground obstructions and structures.It is also well known in the art that detecting objects of interest,such as an underground boring tool 24, is often made difficult orimpossible due to the detection of return signals emanating from manysources not of interest, collectively known as clutter, associated withother underground obstructions, structures, and varying ground mediumcharacteristics, for example. The clutter signal represents backgroundnoise in the composite return signal above which a return signal ofinterest must be distinguished. Attempting to detect the presence of theunderground boring tool 24 using a conventional approach often rendersthe boring tool 24 undetectable or indistinguishable from the backgroundnoise.

It is understood that the return signal from an underground object ofinterest using conventional detection techniques may be weak relative tothe clutter signal content. In such a case, the signal-to-clutter ratiowould be low, which reduces the ability to clearly detect the returnsignal emanating from the underground object of interest. The probe anddetection apparatus and method of the present invention advantageouslyprovides for the production of a return signal from the cooperativetarget 20 provided at the underground boring tool 24 having acharacteristic signature which can be more easily distinguished from theclutter. As will be discussed in detail hereinbelow, the generation of asignature signal containing either or both location and orientationinformation by the cooperative target 20 may be performed eitherpassively or actively.

FIG. 1 illustrates a cross-section through a portion of ground 10 wherethe boring operation takes place, with most of the components of thedetection system depicted situated above the ground surface 11. Thetrenchless underground boring system, generally shown as the system 12,includes a platform 14 on which is situated a tilted longitudinal member16. The platform 14 is secured to the ground by pins 18 or otherrestraining members in order to prevent the platform 14 from movingduring the boring operation. Located on the longitudinal member 16 is athrust/pullback pump 17 for driving a drill string 22 in a forward,longitudinal direction as generally shown by the arrow. The drill string22 is made up of a number of drill string members 23 attachedend-to-end. Also located on the tilted longitudinal member 16, andmounted to permit movement along the longitudinal member 16, is arotating motor 19 for rotating the drill string 22 (illustrated in anintermediate position between an upper position 19 a and a lowerposition 19 b). In operation, the rotating motor 19 rotates the drillstring 22 which has a boring tool 24 at the end of the drill string 22.

A typical boring operation takes place as follows. The rotating motor 19is initially positioned in an upper location 19 a and rotates the drillstring 22. While the boring tool 24 is rotated, the rotating motor 19and drill string 17 are pushed in a forward direction by thethrust-pullback pump 20 toward a lower position into the ground, thuscreating a borehole 26. The rotating motor 19 reaches a lower position19 b when the drill string 22 has been pushed into the borehole 26 bythe length of one drill string member 23. A new drill string member 23is then added to the drill string 22 either manually or automatically,and the rotating motor 19 is released and pulled back to the upperlocation 19 a. The rotating motor 19 then clamps on to the new drillstring member 23 and the rotation/push process is repeated so as toforce the newly lengthened drill string 22 further into the ground,thereby extending the borehole 26. Commonly, water is pumped through thedrill string 22 and back up through the borehole to remove cuttings,dirt, and other debris. If the boring tool 24 incorporates a directionalsteering capability for controlling its direction, a desired directioncan be imparted to the resulting borehole 26.

In FIG. 1, there is illustrated a borehole 26 which bends in thevicinity of a point 31 after the initial oblique section becomesparallel to the ground surface 11. Located above the surface 11, anddetachable from the trenchless underground boring system 12, is aprobing and detection unit 28 (PDU), mounted on wheels 29 or tracks inorder to permit above-ground traversing of the PDU 28 along a pathcorresponding to the underground path of the boring tool 24. The PDU 28is coupled to a control unit 32 via a data transmission link 34.

The operation of the PDU 28 is more clearly described in reference toFIG. 2. The PDU 28 is generally used to transmit a probe signal 36 intothe ground and to detect returning signals. The PDU 28 contains agenerator 52 for generating the probe signal 36 which probes the ground10. A transmitter 54 receives the probe signal 36 from the generator 52,which, in turn, transmits the probe signal 36 (shown as continuous linesin FIG. 2) into the ground 10. In a first embodiment, the generator 52is a microwave generator and the transmitter 54 is a microwave antennafor transmitting microwave probe signals. In an alternative embodiment,the generator 52 is an acoustic wave generator and produces acousticwaves, and the transmitter 54 is typically a probe placed into theground 10 to provide for good mechanical contact for transmitting theacoustic waves into the ground 10.

The probe signal 36 is transmitted by the PDU 28, propagates through theground 10, and encounters subsurface obstructions, one of which is shownas 30, which scatter a return signal 40 (shown as dotted lines in FIG.2) back to the PDU 28. A signature signal 38 (shown as dashed lines inFIG. 2) is also returned to the PDU 28 from the boring tool 24 locatedin the borehole 26.

The detection section of the PDU 28 includes a receiver 56, a detector58, and a signal processor 60. The receiver 56 receives the returnsignals from the ground 10 and communicates them to the detector 58. Thedetector 58 converts the return signals into electric signals which aresubsequently analyzed in the signal processing unit 60. In the firstembodiment described hereinabove in which the probe signal 36constitutes a microwave signal, the receiver 56 typically includes anantenna, and the detector 58 typically includes a detection diode. Inanother embodiment in which the probe signal 36 constitutes an acousticwave, the receiver 56 typically is a probe in good mechanical contactwith the ground 10 and the detector 58 includes a sound-to-electricaltransducer, such as microphone. The signal processor 60 may includevarious preliminary components, such as a signal amplifier, a filteringcircuit, and an analog-to-digital converter, followed by more complexcircuitry for producing a two or three dimensional image of a subsurfacevolume which incorporates the various underground obstructions 30 andthe boring tool 24. The PDU 28 also contains a beacon receiver/analyzer61 for detecting and interpreting a signal from an underground activebeacon. The function of the beacon receiver/analyzer 61 will bedescribed more fully hereinbelow.

The PDU 28 also contains a decoder 63 for decoding information signalcontent that may be encoded on the signature signal produced by thecooperative target 20. Orientation, pressure, and temperatureinformation, for example, may be sensed by appropriate sensors providedin the cooperative target 20, such as a strain gauge for sensingpressure. Such information may be encoded on the signature signal, suchas by modulating the signature signal with an information signal, orotherwise transmitted as part of, or separate from, the signaturesignal. When received by the receiver 56, an encoded return signal isdecoded by the decoder 61 to extract the information signal content fromthe signature signal content. It is noted that the components of the PDU28 illustrated in FIG. 2 need not be contained within the same housingor supporting structure.

Referring once again to FIG. 1, the PDU 28 transmits acquiredinformation along the data transmission link 34 to the control unit 32,which is illustrated as being located in proximity to the trenchlessunderground boring system 12. The data transmission link 34 is providedto handle the transfer of data between the PDU 28 and the trenchlessunderground boring system 12, and may be a co-axial cable, an opticalfiber, a free-space link for infrared communication, or some othersuitable data transfer medium or technique. A significant advantage ofusing a trenchless underground boring system 12 which employs thesubsurface detection technique described herein concerns the detectionof other important subsurface features which may purposefully be avoidedby the boring tool 24, particularly buried utilities such as electric,water, gas, sewer, telephone lines, cable lines, and the like.

Signature signal generation, in accordance with the embodiments of FIGS.3 and 4, may be accomplished using temporal and frequency basedtechniques, respectively. FIG. 3 is an illustration depicting thegeneration and detection of an underground boring tool signature signalin the time domain. Line A shows the emission of a probe signal 36 a asa function of signal character plotted against time. Line B shows areturn signal 62 a detected by the PDU 28 in the absence of anysignature signal generation. The return signal 62 a is depictive of asignal received by the PDU 28 at a time ΔT1 after emission of the probesignal 36 a, and is represented as a commixture of signals returned fromthe underground structure 22 and other scatterers. As previouslydiscussed, a low signal-to-clutter ratio makes it very difficult todistinguish the return signal from the underground boring tool 24.

Line C illustrates an advantageous detection technique in whichcooperation between the cooperative target 22, provided at the boringtool 24, and the PDU 28 is employed to produce and transmit a signaturesignal at a certain time ΔT2 following illumination with the probesignal 36 a. In accordance with this detection scheme, the return signal40 a received from the scatterers is detected initially, and thesignature signal 38 a received from the underground boring tool 24 isdetected after a delay of ΔT2. The delay time ΔT2 is established to besufficiently long so that the signature signal produced by thecooperative target 20 is significantly more pronounced than the cluttersignal at the time of detection. In this case, the signal-to-clutterratio of the signature signal 38 a is relatively high, thus enabling thesignature signal 38 a to be easily distinguished from the backgroundclutter 40 a.

FIG. 4 is an illustration depicting the detection of a cooperativetarget signature signal emitted from an underground boring tool 24 inthe frequency domain. Line A illustrates the frequency band 36 b of theprobe signal as a function of signal strength plotted against frequency.Line B shows a frequency band 62 b of a return signal received from theunderground boring tool 24 in the absence of any cooperative signalgeneration. It can be seen that the naturally occurring return signalsfrom the underground boring tool 24 and other scatterers 30 share afrequency band 62 b similar to that of the probe signal 36 b. Line Cillustrates a case where cooperation is employed between the cooperativetarget 20 of the underground boring tool 24 and the PDU 28 to produceand transmit a signature signal which has a frequency band 38 bdifferent from that of the scattered return signal 40 b. The differencein frequency band, indicated as Δf, is sufficiently large to move thecooperative target signature signal out of, or at least partiallybeyond, the scattered signal frequency band 40 b. Thus, the cooperativetarget signature signal can be detected with relative ease due to theincreased signal-to-clutter ratio. It is noted that high pass, low pass,and notch filtering techniques, for example, or other filtering andsignal processing methods may be employed to enhance cooperative targetsignature signal detection.

It is an important feature of the invention that the underground boringtool 24 be provided with a signature signal-generating apparatus, suchas a cooperative target 20, which produces a signature signal inresponse to a probe signal transmitted by the PDU 28. If no suchsignature signal was produced by the generating apparatus, the PDU 28would receive an echo from the underground boring tool 24 which would bevery difficult to distinguish from the clutter with a high degree ofcertainty using conventional detecting techniques. The incorporation ofa signature signal generating apparatus advantageously provides for theproduction of a unique signal by the underground boring tool 24 that iseasily distinguishable from the clutter and has a relatively highsignal-to-clutter ratio. As discussed briefly above, an active orpassive approach is suitable for generating the boring tool signaturesignal. It is understood that an active signature signal circuit is onein which the circuit used to generate the signature signal requires theapplication of electrical power from an external source, such as abattery, to make it operable. A passive circuit, in contrast, is onewhich does not utilize an external source of power. The source of energyfor the electrical signals present in a passive circuit is the receivedprobe signal itself.

In accordance with a passive approach, the cooperative target 20 doesnot include an active apparatus for generating or amplifying a signal,and is therefore generally less complex than an active approach since itdoes not require the presence of a permanent or replaceable power sourceor, in many cases, electronic circuitry. Alternatively, an activeapproach may be employed which has the advantage that it is moreflexible and provides the opportunity to produce a wider range ofsignature response signals which may be more identifiable whenencountering different types of ground medium. Further, an activeapproach reduces the complexity and cost of manufacturing thecooperative target 20, and may reduce the complexity and cost of thesignature signal receiving apparatus.

Three embodiments of a passive signature signal generating apparatusassociated with a microwave detection technique are illustrated in FIG.5. Each of the embodiment illustrations shown in FIG. 5 includes aschematic of a cooperative target 20 including a microwave antenna andcircuit components which are used to generate the signature signal. Thethree embodiments illustrated in FIGS. 5 a, 5 b, and 5 c are directedtoward the generation of the signature signal using a) the time domain,b) the frequency domain and c) cross-polarization, respectively.

In FIG. 5 a, there is illustrated a cooperative target 20 which includestwo antennae, a probe signal receive antenna 66 a, and a signaturesignal transmit antenna 68 a. For purposes of illustration, theseantennae are illustrated as separate elements, but it is understood thatmicrowave transmit/receive systems can operate using a single antennafor both reception and transmission. Two separate antennae are used inthe illustration of this and the following embodiments in order toenhance the understanding of the invention and, as such, no limitationof the invention is to be inferred therefrom. The receive antenna 66 aand the transmit antenna 68 a in the physical embodiment of thesignature signal generator will preferably be located inside thecooperative target 20 or on its surface in a conformal configuration.For antennae located entirely within the cooperative target 20, it isunderstood that at least a portion of the cooperative target housing ismade of a non-metallic material, preferably a hard dielectric material,thus allowing passage of the microwaves through at least a portion ofthe cooperative target housing. A material suitable for this applicationis KEVLAR®. Antennae that extend outside of the cooperative targethousing may be covered by a protective non-metallic material. Theantennae, in this configuration, may be made to conform to the housingcontour, or disposed in recesses provided in the housing and coveredwith an epoxy material, for example.

The illustration of FIG. 5 a shows the signature signal generationapparatus for a microwave detection system operating in the time domain.In accordance with this embodiment, a receive antenna 66 a receives aprobe signal 70 a from the PDU 28, such as a short microwave burstlasting a few nanoseconds, for example. In order to distinguish asignature signal 74 a from the clutter received by the PDU 28, thereceived probe signal 70 a passes from the receive antenna 66 a into atime-delaying waveguide 72 a, preferably a co-axial cable, to a transmitantenna 68 a. The signature signal 74 a is then radiated from thetransmit antenna 68 a and received by the PDU 28. The use of thetime-delay line, which preferably delays the response from thecooperative target 20 by about 10 nanoseconds, delays radiating thereturn signature signal 74 a until after the clutter signal received bythe PDU 28 has decreased in magnitude.

In accordance with another embodiment, a single antenna embodiment ofthe passive time domain signature generator could be implemented bycutting the waveguide at the point indicated by the dotted line 76 a toform a termination. In this latter embodiment, the probe signal 70 apropagates along the waveguide 72 a until it is reflected by thetermination located at the cut 76 a, propagates back to the receiveantenna 66 a, and is transmitted back to the PDU 28. The terminationcould be implemented either as an electrical short, in which case theprobe signal 70 a would be inverted upon reflection, or as an opencircuit, in which case the probe signal 70 a would not be inverted uponreflection.

The introduction of a time delay to create the signature signal 74 amakes the underground boring tool 24 appear deeper in the ground than itis in actuality. Since microwaves are heavily attenuated by the ground,ground penetrating radar systems have a typical effective depth range ofabout 10 feet when employing conventional detection techniques, beyondwhich point the signal returns are generally too heavily attenuated tobe reliably detected. The production of a time delayed signature signalreturn 74 a from the underground boring tool 24 artificially translatesthe depth of the underground boring tool 24 to an apparent depth in therange of 10 to 20 feet, a depth from which there is generally no otherstrong signal return, thus significantly enhancing the signal-to-clutterratio of the detected signature signal 74 a. The actual depth of theunderground boring tool 24 may then be determined by factoring out theartificial depth component due to the known time delay associated withthe cooperative target 20. It is believed that the signature signalgenerated by a cooperative target 20 may be detectable at actual depthson the order of 100 feet. It is further believed that a signature signalgenerated by an active device will generally be stronger, and thereforemore detectable, than a signature signal produced by a passive device.

The illustration of FIG. 5 b depicts a signature signal generatingapparatus for a microwave detection system operating in the frequencydomain. In accordance with this embodiment, a receive antenna 66 b,provided in or on the boring tool 24, receives a microwave probe signal70 b from the PDU 28. The probe signal 70 b is preferably a microwaveburst, lasting for several microseconds, which is centered on a givenfrequency, f, and has a bandwidth of Δf1, where Δf1/f is typically lessthan one percent. In order to shift a return signature signal 74 b outof the frequency regime associated with the clutter received by the PDU28, the received probe signal 70 b propagates from the receive antenna66 b along a waveguide 72 b into a nonlinear device 78 b, preferably adiode, which generates harmonic signals, such as second and thirdharmonics, from an original signal.

The harmonic signal is then radiated from a transmit antenna 68 b as thesignature signal 74 b and is received by the PDU 28. The PDU 28 is tunedto detect a harmonic frequency of the probe signal 70 b. For a probesignal 70 b of 100 MHz, for example, a second harmonic detector 58 wouldbe tuned to 200 MHz. Generally, scatterers are linear in their responsebehavior and generate a clutter signal only at a frequency equal to thatof the probe signal 70 b. Since there is generally no other source ofthe harmonic frequency present, the signal-to-clutter ratio of thesignature signal 74 b at the harmonic frequency is relatively high. In amanner similar to that discussed hereinabove with respect to the passivetime domain embodiment, the passive frequency domain embodiment may beimplemented using a single antenna by cutting the waveguide at the pointindicated by the dotted line 76 b to form a termination. In accordancewith this latter embodiment, the probe signal 70 b would propagate alongthe waveguide 72 b, through the nonlinear element 78 b, reflect at thetermination 76 b, propagate back through the nonlinear element 78 b,propagate back to the receive antenna 66 b, and be transmitted back tothe PDU 28. The polarity of the reflection would be determined by thenature of the termination, as discussed hereinabove.

The illustration of FIG. 5 c depicts signature signal generation for amicrowave detection system operating in a cross-polarization mode. Inaccordance with this embodiment, the PDU 28 generates a probe signal 70c of a specific linear polarity which is then transmitted into theground. The clutter signal is made up of signal returns from scattererswhich, in general, maintain the same polarization as that of the probesignal 70 c. Thus, the clutter signal has essentially the samepolarization as the probe signal 70 c. A signature signal 74 c isgenerated in the cooperative target 20 by receiving the polarized probesignal 70 c in a receive antenna 66 c, propagating the signal through awaveguide 72 c to a transmit antenna 68 c, and transmitting thesignature signal 74 c back to the PDU 28. The transmit antenna 68 c isoriented so that the polarization of the radiated signature signal 74 cis orthogonal to that of the received probe signal 70 c. The PDU 28 mayalso be configured to preferentially receive a signal whose polarizationis orthogonal to that of the probe signal 70 c. As such, the receiver 56preferentially detects the signature signal 74 c over the cluttersignal, thus improving the signature signal-to-clutter ratio.

In a manner similar to that discussed hereinabove with respect to thepassive time and frequency domain embodiments, the cross-polarizationmode embodiment may be implemented using a single antenna by cutting thewaveguide at the point indicated by the dotted line 76 c to form atermination and inserting a polarization mixer 78 c which alters thepolarization of the wave passing therethrough. In this latterembodiment, the probe signal would propagate along the waveguide 72 c,through the polarization mixer 78 c, reflect at the termination 76 c,propagate back through the polarization mixer 78 c, propagate back tothe receive antenna 66 c and be transmitted back to the PDU 28. Thepolarity of the reflection may be determined by the nature of thetermination, as discussed previously hereinabove. It is understood thatan antenna employed in the single antenna embodiment would be requiredto have efficient radiation characteristics for orthogonalpolarizations. It is further understood that the cross-polarizationembodiment may employ circularly or elliptically polarized microwaveradiation. It is also understood that the cross-polarization embodimentmay be used in concert with either the passive time domain or passivefrequency domain signature generation embodiments described previouslywith reference to FIGS. 5 a and 5 b in order to further enhance thesignal-to-clutter ratio of the detected signature signal.

Referring now to FIG. 6, active signature signal generation embodimentswill be described. FIG. 6 a illustrates an embodiment of active timedomain signature signal generation suitable for incorporation in aboring tool 24. The embodiment illustrated shows a probe signal 82 abeing received by a receive antenna 84 a which is coupled to adelay-line waveguide 86 a. An amplifier 88 a is located at a point alongthe waveguide 86 a, and amplifies the probe signal 82 a as it propagatesalong the waveguide 86 a. The amplified probe signal continues along thedelay-line waveguide 86 a to the transmit antenna 90 a which, in turn,transmits the signature signal 92 a back to the PDU 28. FIG. 6 billustrates an alternative embodiment of the active time domainsignature generator which incorporates a triggerable delay circuit forproducing the time-delay, rather than propagating a signal along alength of time-delay waveguide. The embodiment illustrated shows a probesignal 82 b being received by a receive antenna 84 b coupled to awaveguide 86 b. A triggerable delay circuit 88 b is located at a pointalong the waveguide 86 b. The triggerable delay circuit 88 b operates inthe following fashion. The triggerable delay circuit 88 b is triggeredby the probe signal 82 b which, upon initial detection of the probesignal 82 b, initiates an internal timer circuit. Once the timer circuithas reached a predetermined delay time, preferably in the range 1-20nanoseconds, the timer circuit generates an output signal from thetriggerable delay circuit 88 b which is used as a signature signal 92 b.The signature signal 92 b propagates along the waveguide 86 b to atransmit antenna 90 b which then transmits the signature signal 92 b tothe PDU 28.

FIG. 6 c illustrates an embodiment of an active frequency domainsignature generator suitable for incorporation in or on an undergroundboring tool 24. The embodiment illustrated shows a probe signal 82 cbeing received by a receive antenna 84 c coupled to a waveguide 86 c anda nonlinear element 88 c. The frequency-shifted signal generated by thenonlinear element 88 c is then passed through an amplifier 94 c beforebeing passed to the transmit antenna 90 c, which transmits the signaturesignal 92 c to the PDU 28. The amplifier 94 c may also include afiltering circuit to produce a filtered signature signal at the outputof the amplifier 94 c. An advantage to using an active frequency domainsignature signal generation embodiment over a passive frequency domainsignature signal generation embodiment is that the active embodimentproduces a stronger signature signal which is more easily detected.

In a second embodiment of the active frequency domain signature signalgenerator, generally illustrated in FIG. 6 c, a probe signal 82 c passesthrough the amplifier 94 c prior to reaching the nonlinear element 88 c.An advantage of this alternative embodiment is that, since theamplification process may take place at a lower frequency, the amplifiermay be less expensive to implement.

A third embodiment of an active frequency domain signature generatorsuitable for use with an underground boring tool 24 is illustrated inFIG. 6 d. FIG. 6 d shows a receive antenna 84 d coupled through use of awaveguide 86 d to a frequency shifter 88 d and a transmit antenna 90 d.The frequency shifter 88 d is a device which produces an output signal92 d having a frequency of f2, which is different from the frequency,f1, of an input signal 82 d by an offset Δf, where f2=f1+Δf. Inaccordance with this embodiment, Δf is preferably larger than one halfof the bandwidth of the probe signal 82 d, typically on the order of 1MHz. The frequency shifter 88 d produces a frequency shift sufficient tomove the signature signal 92 d out of, or at least partially beyond, thefrequency band of the clutter signal, thereby increasing thesignal-to-clutter ratio of the detected signature signal 92 d. Forpurposes of describing these embodiments, the term signature signalembraces all generated return signals from the cooperative target 20other than those solely due to the natural reflection of the probesignal off of the underground boring tool 24. FIG. 7 illustrates anembodiment of a signature signal generator adapted for use in acooperative target 20 provided on or within an underground boring tool24 where the probe signal is an acoustic signal. In an acoustictime-domain embodiment, as illustrated in FIG. 7 a, an acoustic probesignal 98 a, preferably an acoustic impulse, is received and detected byan acoustic receiver 100 a mounted on the inner wall 96 a of the boringtool 24. The acoustic receiver 100 a transmits a trigger signal along atrigger line 102 a to a delay pulse generator 104 a. After beingtriggered, the delay pulse generator 104 a generates a signature pulsefollowing a triggered delay. The signature pulse is passed along thetransmitting line 106 a to an acoustic transmitter 108 a, also mountedon the inner wall 96 a of the boring tool 24. The acoustic transmitter108 a then transmits an acoustic signature signal 110 a through theground for detection by the PDU 28.

In accordance with an acoustic frequency-domain embodiment, as isillustrated in FIG. 7 b, an acoustic probe signal 98 b, preferably anacoustic pulse having a given acoustic frequency f3, is received anddetected by an acoustic receiver 100 b mounted on the inner wall 96 b ofthe boring tool 24. The acoustic receiver 100 b transmits an inputelectrical signal corresponding to the received acoustic signal 98 b ata frequency f3 along a receive line 102 b to a frequency shifter 104 b.The frequency shifter 104 b generates an output electrical signal havinga frequency that is shifted by an amount Δf3 relative to the frequencyof the input signal 98 b. The output signal from the frequency shifter104 b is passed along a transmit line 106 b to an acoustic transmitter108 b, also mounted on the inner wall 96 b of the boring tool 24. Theacoustic transmitter 108 b then transmits the frequency shifted acousticsignature signal 110 b through the ground for detection by the PDU 28.

In FIG. 8, there is illustrated in system block diagram form anotherapparatus for actively generating in a cooperative target 20 a signaturesignal that contains various types of information content. In oneconfiguration, the signature signal generating apparatus of thecooperative target 20 includes a receive antenna 41, a signature signalgenerator 43, and a transmit antenna 45. In accordance with thisconfiguration, a probe signal 37 produced by the PDU 28 is received bythe receive antenna 41 and transmitted to a signature signal generator43. The signature signal generator 43 alters the received probed signal37 so as to produce a signature signal that, when transmitted by thetransmit antenna 45, is readily distinguishable from other return andclutter signals received by the PDU 28. Alternatively, the signaturesignal generator 43, in response to the received probe signal 37,generates a signature signal different in character than the receivedprobe signal 37. The signature signal transmitted by the transmitantenna 45 differs from the received probe signal 37 in one or morecharacteristics so as to be readily distinguishable from other returnand clutter signals. By way of example, and as discussed in detailhereinabove, the signature signal produced by the signature signalgenerator 43 may differ in phase, frequency content, polarization, orinformation content with respect to other return and clutter signalsreceived by the PDU 28.

Additionally, as is further illustrated in FIG. 8, the cooperativetarget 20 may include an orientation detector 47. The orientationdetector 47 is a device capable of sensing an orientation of thecooperative target 20, and provides an indication of the orientation ofthe underground boring tool 24 during operation.

It may be desirable for the operator to know the orientation of theboring tool 24 when adjusting the direction of the boring tool 24 alongan underground pathway, since several techniques known in the art fordirecting boring tools rely on a preferential orientation of the tool.If the boring tool 24 orientation is not known, the boring tool 24cannot be steered in a preferred direction in accordance with such knowntechniques that require knowledge of boring tool 24 orientation. It maynot be possible to determine the orientation of the boring tool 24simply from a knowledge of the orientation of the members 23 of thedrill string 22, since one or more members 23 of the drill string 22 maytwist or slip relative to one another during the boring operation. Sincethe boring operation takes place underground, the operator has no way ofdetecting whether such twisting or slipping has occurred. It may,therefore, be important to determine the orientation of the boring tool24.

The orientation detector 47 produces an orientation signal which iscommunicated to an encoder 49, such as a signal summing device, whichencodes the orientation signal produced by the orientation detector 47on the signature signal produced by the signature signal generator 43.

The encoded signature signal produced at the output of the encoder 49 iscommunicated to the transmit antenna 45 which, in turn, transmits theencoded signature signal 39 to the PDU 28. Various known techniques forencoding the orientation signal on the signature signal may beimplemented by the encoder 49, such as by modulating the signaturesignal with the orientation signal. It is noted that other sensors maybe included within the apparatus illustrated in FIG. 8 such as, forexample, a temperature sensor or a pressure sensor. The outputs of suchsensors may be communicated to the encoder 49 and similarly encoded onthe signature signal for transmission to the PDU 28 or, alternatively,may be transmitted as information signals independent from the signaturesignal.

Referring to FIG. 9, there is illustrated an embodiment of anorientation detecting apparatus which may include up to three mutuallyorthogonally arranged orientation detectors. The orientation detectors210, 212, and 214 are aligned along the x-axis, y-axis, and z-axis,respectively. In accordance with this embodiment, the orientationdetector 210 detects changes in orientation with respect to the x-axis,while the orientation detector 212 senses changes in orientation withrespect to the y-axis. Similarly, the orientation detector 214 detectschanges in orientation with respect to the z-axis. Given thisarrangement, changes in pitch, yaw, and roll may be detected when thecooperative target 20 is subject to positional changes. It is noted thata single orientation detector, such as detector 210, may be used tosense changes along a single axis, such as pitch changes in the boringtool 24, if multiple axis orientation changes need not be detected.Further, depending on the initial orientation of the cooperative target20 when mounted to the underground boring tool 24, two orthogonallyarranged orientation detectors, such as orientation detectors 210 and212 aligned respectively along the x and y-axes, may be sufficient toprovide pitch, yaw, and roll information.

Referring now to FIG. 10, there is illustrated an embodiment of anapparatus for detecting an orientation of an underground boring tool 24.In accordance with this embodiment, the cooperative target 20 providedon or within the underground boring tool 24 includes a tilt detector 290that detects changes in boring tool orientation during boring activity.The cooperative target 20, in addition to producing a signature signalfor purposes of determining boring tool location, may include anorientation detector, such as that illustrated in FIG. 10, for purposesof producing and orientation signal representative of an orientation ofthe cooperative target 20 and, therefore, the underground boring tool24.

In one embodiment, as is illustrated in FIG. 8, the cooperative target20 includes an orientation detecting apparatus, which produces anorientation signal, and a separate signature signal generator, whichproduces a signature signal. The signature signal and the orientationsignal may be transmitted by the transmit antenna 45 of the cooperativetarget 20 as two separate information signals or, alternatively, as acomposite signal which includes both the signature and orientationsignals. Alternatively, the orientation detecting apparatus may producea single signature signal that is indicative of both the location andthe orientation of the cooperative target 20.

Referring in greater detail to FIG. 10, there is illustrated a tiltdetector 290 coupled to a selector 291. The tilt detector 290 detectstilting of the cooperative target 20 with respect to one or moremutually orthogonal axes of the boring tool 24. It is believed that thetilt detector 290 illustrated in FIG. 10 is useful as a sensor thatsenses the pitch of the boring tool 24 during operation. The range oftilt angles detectable by the tilt detector 290 may be selected inaccordance with the estimated amount of expected boring tool tilting fora given application. For example, the tilt detector 290 may detectmaximum pitch angles in the range of ±45° relative to horizontal in oneapplication, whereas, in another application, the tilt detector 290 maydetect pitch angles in the range of ±90° relative to horizontal, forexample. It is to be understood that the tilt detector 290, as well asother components illustrated in FIG. 10, may be active or passivecomponents.

As is further illustrated in FIG. 10, a probe signal 235 is received bythe receive antenna 234 which, in turn, communicates the probe signal235 to a selector 291. The tilt detector 290 and selector 291 cooperateto select one of several orientation signal generators depending on themagnitude of tilting as detected by the tilt detector 290. In oneembodiment, the probe signal 235 is coupled to each of the orientationsignal generators P₁ 292 through P_(N) 297, one of which is selectivelyactivated by the tilt detector 290 which incorporates the function ofthe selector 291, such as the embodiment illustrated in FIG. 12. Inanother embodiment, the probe signal 235 is coupled to the selector 291which activates one of the orientation signal generators P₁ 292 throughP_(N) 297 depending on the magnitude of tilting detected by the tiltdetector 290.

By way of example, and in accordance with a passive componentimplementation, each of the orientation signal generators P₁ 292 throughP_(N) 297 represent individual transmission lines, each of whichproduces a unique time-delayed signature signal which, when transmittedby the transmit antenna 244, provides both location and orientationinformation when received by the PDU 28. As such, the orientationdetection apparatus in accordance with this embodiment provides bothlocation and orientation information and does not require a separatesignature signal generator 43. In another embodiment, each of theorientation signal generators, such as orientation signal generator P₃294, produces a unique orientation signal which is transmitted to anencoder 49. A signature signal 299 produced by a signature signalgenerator 43 separate from the orientation detection apparatus may beinput to the encoder 49, which, in turn, produces a composite signaturesignal 301 which includes both signature signal and orientation signalcontent. The composite signal 301 is then transmitted to the PDU 28 anddecoded to extract the orientation signal content from the signaturesignal content.

As discussed previously, the range of tilt angles detectable by the tiltdetector 290 and the resolution between tilt angle increments may varydepending on a particular application or use. By way of example, it isassumed that the tilt detector 290 is capable of detecting maximum tiltangles of ±60°. The selector 291 may select orientation signal generatorP₁ 292 when the tilt detector 290 is at a level or null state (i.e., 0°tilt angle) relative to horizontal. When selected, orientation detectorP₁ 292 generates a unique orientation signal which is indicative of anorientation of 0°. As previously discussed, the orientation signal maybe combined with a signature signal produced by a separate signaturesignal generator 43 or, alternatively, may provide both signature signaland orientation signal information which is transmitted to the PDU 28.

In the event that the tilt detector 290 detects a positive 5° tilt anglechange, for example, orientation signal generator P₂ 293 is selected bythe selector 291. The orientation signal generator P₂ 293 then producesan orientation signal that indicates a positive 5° tilt condition.Similarly, orientation signal generators P₃ 294, P₄ 295, and P₅ 296 mayproduce orientation signals representing detected tilt angle changes ofpositive 10°, 15°, and 20°, respectively. Other orientation signalgenerators may be selected by the selector 291 to produce orientationsignals representing tilt angle changes in five degree incrementsbetween 25° and 60°. Negative tilt angles between 0° and −60° in 5°increments are preferably communicated to the PDU 28 by selection ofappropriate orientation signal generators corresponding to the magnitudeof negative tilting. It will be appreciated that the range andresolution between tilt angle increments may vary depending on aparticular application.

In FIGS. 11 a and 11 b, there is illustrated another embodiment of anunderground boring tool 500 equipped with a signature signal generatingapparatus which, in addition to providing location information, providesboring tool orientation information. Referring to FIG. 11 a, the boringtool 500 includes a longitudinal axis 501 about which the boring tool500 rotates during boring activity. Distributed about the periphery ofthe boring tool 500 are a number of a signature signal generatingdevices, such as devices 504 and 508. In accordance with thisembodiment, the signature signal generating devices operate passivelyand, as such, do not require an external power supply. Each of thesignature signal generating devices distributed about the boring tool500 produces a unique signature signal in response to a received probesignal generated by the PDU 28.

As is further illustrated in FIG. 11 b, the boring tool 500 includes anumber of elongated recesses or channels within which signature signalgenerating devices are disposed. In FIG. 11 b, there is shown across-sectional view of the boring tool 500 illustrated in FIG. 11 a. Asignature signal generating device 504, such as a co-axial transmissionline, for example, is disposed in a recess 502 and encased in aprotective material 505 which permits passage of electromagnetic signalstherethrough. The protective material 505 fixes the signature signalgenerating device 504 within the channel 502. Also shown in FIG. 11 b isa second signature signal generating device 508 similarly disposed in arecess 506 and encased in a protective material 505. A hard dielectricmaterial, such as KEVLAR®, is a material suitable material for thisapplication.

During operation, the boring tool 500 is rotated at an appropriatedrilling rate which, assuming a full 360° rotation, exposes each of thesignature signal generating devices to a probe signal 36 produced by thePDU 28. When exposed to the probe signal 36 during rotation, each of thesignature signal generating devices will emit a characteristic orsignature signal 38 in response to the probe signal 36. As a particularsignature signal generating device rotates beyond a reception windowwithin which the probe signal 36 is received and a signature signal 38generated, the bulk metallic material of the boring tool 500 shieldssuch a signature signal generating device from the probe signal 36. Itmay be desirable to situate the signature signal generating devicesabout the periphery of the boring tool 500 such that the signaturesignal produced by the signature signal detecting device exposed to theprobe signal 36 produces the predominant signature signal 38 received bythe PDU 28. It may further be desirable to provide for a null or deadzone between adjacent signature signal generating devices so that theonly signature signal 38 received by the PDU 28 is that produced by asingle signature signal generating device currently exposed to the probesignal 38.

The type of signature signal generating device, configuration of theboring tool recesses, such as recess 502, the type of protectivematerial 505 employed, the number and location of signature signalgenerating devices used, and the rotation rate of the boring tool 500will typically impact the ability of the PDU 28 to detect the signaturesignal 38 produced by each of the signature signal generating devicesduring boring tool rotation.

Turning now to FIG. 12, there is illustrated an embodiment of anorientation detector suitable for use in both active and passivesignature signal generating apparatuses. In one embodiment, a mercurysensor 220 may be constructed having a bent tube 221 within which a beadof mercury 222 moves as the tube 221 tilts within a plane defined by theaxes 223 and 225. Pairs of electrical contacts, such as contacts 227 and229, are distributed along the base of the tube 221. As the tube 221tilts, the mercury bead 222 is displaced from an initial or null point,generally located at a minimum bend angle of the tube 221. As the bead222 moves along the tube base, electrical contact is made betweenelectrical contact pairs 227 and 229 distributed along the tube base. Asthe amount of tube tilting increases, the mercury bead 222 is displacedfurther from the null point, thus completing electrical circuit pathsfor contact pairs located at corresponding further distances from thenull point. As such, the incremental change in tilt magnitude may bedetermined by detecting continuity in the contact pair over which themercury bead 222 is situated.

In one embodiment, sixty-four of such contact pairs are provided alongthe base of the tube 221 to provide 64-bit tilt resolution information.An electrical circuit or logic (not shown) is coupled to the pairs ofelectrical contacts 227 and 229 which provides an output indicative ofthe magnitude of tube tilting, and thus an indication of the magnitudeof the cooperative target orientation with respect to the plane definedby axes 223 and 225. It is appreciated that use of a mercury sensor 220in accordance with this embodiment may require a power source. As such,this embodiment of an orientation detector is appropriate for use inactive signature signal generating circuits. It is noted that the rangeof tilt angles detectable by the mercury sensor 220 is dependent on thebend angle a provided in the bent tube 221. The bend angle a, as well asthe length of the tube 221, will also impact the detection resolution ofmercury bead displacement within the tube 221.

In accordance with another embodiment of an orientation detectorsuitable for use in passive signature signal generating circuits,reference is made to FIGS. 12 and 13 a-13 b. The illustration of theapparatus depicted in FIG. 12 may be viewed in a context other than thatpreviously described in connection with a mercury sensor embodiment. Inparticular, a metallic ball or other metallic object 222 is displacedwithin a tube 221 in response to tilting of the tube 221 within theplane defined by the axes 223 and 225. The movable contact 222 movesalong a pair of contact rails 235 a and 235 b separated by a channel237. The rails 235 a and 235 b include gaps 233 which separate onecontact rail circuit from an adjacent contact rail circuit. As isillustrated in detail in FIGS. 13 a-13 b, each of the contact railcircuits is coupled to a pair of contacts 227 and 229 which, in turn,are coupled to a transmission line capable of producing a uniquesignature signal.

By way of example, and with particular reference to FIGS. 13 a-13 b,movable contact 222 is shown moving within the tube 221 between a firstposition P_(a) and a second position P_(b) in response to tilting of thetube 221. When the movable contact 222 is at the position P_(a),continuity is established between contact 227, contact rail 235 a,movable contact 222, contact rail 235 b, and contact 229. As such, thecircuit path including the transmission line T₄ 230 is closed. A probesignal 235 produced by the PDU 28 is received by the receive antenna 234which communicates the probe signal along an input waveguide 232 andthrough the circuit path defined by contact 227, rail contact 235 a,movable contact 222, rail contact 235 b, and contact 229. The receivedprobe signal 235 transmitted to the time-delaying waveguide T₄ 230produces a time-delayed signature signal which is communicated to anoutput waveguide 242 and to a transmit antenna 244. The signature signalproduced by the waveguide T₄ 230 is then received by the PDU 28. The PDU28 correlates the signature signal 245 with the selected signaturesignal waveguide, such as transmission line T₄ 230, and determines themagnitude of tube 221 tilting. Those skilled in the art will appreciatethat various impedance matching techniques, such as use of quarterwavelength matching stubs and the like, may be employed to improveimpedance matching within the waveguide pathways illustrated in FIGS. 13a-13 b.

Referring now to FIG. 14, there is illustrated another embodiment of anorientation detection apparatus suitable for detecting an orientation ofan underground boring tool 510. In accordance with this embodiment, anumber of rotation detectors, such as R₁ 512 and R₂ 514, are disposed atvarious radial locations about the periphery of the boring tool 510. Therotation detectors detect radial displacement of the boring tool 510 asthe boring tool 510 rotates about its longitudinal axis 501. A pitchdetector 516, oriented parallel with the longitudinal axis 501 of theboring tool 510, is susceptible to changes in boring tool pitch. In oneembodiment, the rotation detectors, such as R₁ 512 and R₂ 514, and thepitch detector 516 are accelerometer-type sensors. Alternatively, therotation and pitch detectors may constitute spring or strain gauge stylesensors. Various other known displacement sensor mechanisms may also beemployed.

The magnitude of the responsive of each rotation detector, such asdetector R₁ 512, is typically dependent on the radial location of aparticular rotation detector relative to earth's gravity vector as theboring tool 24 rotates about the longitudinal axis 501. The magnitude ofthe output produced by the pitch detector 516 is typically dependent onthe degree of a boring tool pitch off of horizontal relative to theground surface 11. The output signal produced by each of the rotationdetectors and the pitch detector may be encoded onto the signaturesignal produced by the signature signal generating apparatus provided onthe boring tool 510 or, alternatively, transmitted to the PDU 28 as aseparate information signal.

FIG. 21 a illustrates yet another embodiment of an orientation sensingapparatus suitable for use with a boring tool 400. The boring tool 400incorporates a passive time domain signature signal circuit including asingle antenna 402, connected via a time delay line 404 to a termination406, as discussed hereinabove with respect to FIG. 5 a. The circuitillustrated in FIG. 21 a also includes a mercury switch 408 located at apoint along the delay line 404 close to the termination 406. Thetermination 406 also includes a dissipative load. When the boring tool400 is oriented so that the mercury switch 408 is open, the time domainsignature signal is generated by reflecting an incoming probe signal 407at the open circuit of the mercury switch 408. When the boring tool 400is oriented so that the mercury switch 408 is closed, the circuit fromthe antenna 402 is completed to the dissipative load 406 through thedelay line 404. The probe signal 407 does not reflect from thedissipative load 406 and therefore no signature signal is generated. Thegeneration of the signature signal 409 received by the PDU 28 is shownas a function of time in FIG. 21 b. The top trace 407 b shows the probesignal 407, I_(p), plotted as a function of time.

As the boring tool 400 rotates and moves along an underground path, theresistance, Rm, of the mercury switch 408 alternates from low to highvalues, as shown in the center trace 408 b. The regular opening andclosing of the mercury switch 408 modulates the signature signal 409 b,I_(s), received at the surface. The modulation maintains a constantphase relative to a preferred orientation of the boring tool 24. Thelower trace does not illustrate the delaying effects of the time delayline 404 since the time scales are so different (the time delay on thesignature signal 409 is of the order of 10 nanoseconds, while the timetaken for a single rotation of the boring tool 24 is typically between0.1 and 1 second). Detection of the modulated signature signal 409 bythe PDU 28 allows the operator to determine the orientation of theboring tool head. It is understood that the other embodiments ofsignature signal generation described hereinabove can also incorporate amercury switch 408 and, preferably, a dissipative load 406 in order togenerate a modulated signature signal 409 for purposes of detecting theorientation of the boring tool 24.

In FIG. 15, there is illustrated an apparatus for actively generating asignature signal and an orientation signal in an underground boring tool24. There is shown the head of a boring tool 24 a. At the front end ofthe boring tool 24 a is a cutter 120 for cutting through soil, sand,clay, and the like when forming an underground passage. A cut-awayportion of the boring tool wall 122 reveals a circuit board 124 which isdesigned to fit inside of the boring tool 24 a. Attached to the circuitboard 124 is a battery 126 for providing electrical power. Alsoconnected to the circuit board 124 is an antenna 128 which is used toreceive an incoming probe signal 36 and transmit an outgoing signaturesignal 38. The antenna 128 may be located inside the boring tool 24 a ormay be of a conformal design located on the surface of the boring tool24 a and conforming to the surface contour. The boring tool 24 a mayalso contain one or more sensors for sensing the environment of theboring tool 24 a. Circuitry is provided in the boring tool 24 a forrelaying this environmental information to the control unit 32 situatedabove-ground. The sensors, such as an orientation sensor 131, may beused to measure, for example, the orientation of the boring tool 24 a,(pitch, yaw, and roll) or other factors, such as the temperature of thecutting tool head or the pressure of water at the boring tool 24 a.

In FIG. 15, there is illustrated a sensor 130, such as a pressuresensor, located behind the cutter 120. An electrical connection 132 runsfrom the sensor 130 to the circuit board 124 which contains circuitryfor analyzing the signal received from the sensor 130. The circuit board124 may modulate the signature signal 38 to contain information relatingto the sensor output or, alternatively, may generate separate sensorsignals which are subsequently detected and analyzed above-ground. Alsodepicted is an orientation sensor 131 which produces an orientationsignal indicative of an orientation of the boring tool 24, such as thelateral position or deviation of the boring tool 24 relative to apredefined underground path or, by way of further example, the pitch ofthe boring tool 24 relative to horizontal.

A methodology for detecting the depth of a boring tool 24 incorporatinga cooperative target 20 in accordance with one embodiment is illustratedin FIG. 16. In accordance with this embodiment, the PDU 28 includes atransmit antenna 250 and two receive antennas, AR₁ 252 and AR₂ 254. Eachof the receive antennas AR₁ 252 and AR₂ 254 is situated a known distance2 m from the transmit antenna AT 250. It is assumed for purposes of thisexample that the propagation rate K through the ground medium ofinterest is locally constant. Although this assumption may introduce adegree of error with respect to actual or absolute depth boring tool,any such error is believed to be acceptable given the typicalapplication or use of the boring tool cooperative detection techniquedescribed here. In other applications, absolute depth determinations maybe desired. In such a case, the local propagation rate K, or dielectricconstant, may be empirically derived, one such procedure being describedhereinbelow.

Returning to FIG. 16, the time-of-flight, t₁, of the signal travelingbetween the cooperative target 20 of the boring tool 24 and the receiveantenna AR 252, and between the transmit antenna 250 and the cooperativetarget 20 of the boring tool 24 is determined when the cooperativetarget 20 is positioned below the centerline of the antennas AR₁ 252 andAT 250. The travel time of the signal traveling between the cooperativetarget 20 and the receive antenna AR₂ 254 is indicated as the time t₂.The depth d of the boring tool 24 that incorporates the cooperativetarget 20 may then be determined by application of the followingequations:d ² =K ²(t ₁ ²)−m ²  [1]d ² =K ²(t ₂ ²)−9m ²  [2]K ²(t ₂ ²)−K ²(t ₁ ²)=8m ²  [3]K ²(t ₂ ² −t ₁ ²)=8m ²  [4]K ²=[8m ²/(t ₂ ² −t ₁ ²)]  [5]d ²=[8m ²/(t ₂ ² −t ₁ ²)](t ₁ ²)−m ²  [6]d=m[(8t ₁ ²/(t ₂ ² −t ₁ ²)) −1]²  [7]

In accordance with an alternative approach for determining the depth dof a cooperative target 20, depth calculations may be based onfield-determined values for characteristic soil properties, such as thedielectric constant and wave velocity through a particular soil type. Asimplified empirical technique that may be used when calibrating thedepth measurement capabilities of a particular GPR system involvescoring a sample target, measuring its depth, and relating it to thenumber of nanoseconds it takes for a wave to propagate through the coresample.

For an embodiment of the invention which uses a microwave probe signal,a general relationship for calculating the depth or dielectric constantfrom the time of flight measurement is described by the followingequation: $\begin{matrix}{{TE} = {{{TF} - {TD}} = {\sum\frac{d_{j}\sqrt{ɛ_{j}}}{c}}}} & \lbrack 8\rbrack\end{matrix}$where, TE is an effective time-of-flight, which is the duration of timeduring which a probe signal or signature signal is traveling through theground; TF is the measured time-of-flight; TD is the delay internal tothe cooperative target between receiving the probe signal andtransmitting the signature signal; d_(j) is the thickness of the jthground type above the cooperative target; M_(j) is the averagedielectric constant of the jth ground type at the microwave frequency;and c is the speed of light in a vacuum. It is important to know thedielectric constant since it provides information related to the type ofsoil being characterized and its water content. Having determined thedielectric constant of a particular soil type, the depth of the boringtool 24 traversing through similar soil types can be directly derived byapplication of the above-described equations.

A methodology for detecting the location of an underground boring tool24 as the boring tool 24 creates or otherwise travels along anunderground path is illustrated in FIGS. 17 a-17 b and 18 a-18 b. Withreference to these figures and to FIG. 1, an underground boringoperation is depicted in which a boring tool 24 is shown excavating theground 10 so as to create an underground path or borehole 26. The drillstring 22 is increased in length during the boring operation typicallyby adding individual drill string members 23 to the drill string 22 in amanner previously discussed. As the drill string length is increased,and the boring tool 24 forced further into the ground 10, the PDU 28 ismoved along a preferred above-ground path 41 at a speed approximatelyequal to the horizontal speed component of the boring tool 24.

In one embodiment, the PDU 28 repeatedly transmits a probe signal 36into the ground 10 when moved along the path 41, which is received bythe signature signal generating apparatus provided on or within theboring tool 24. In response to the probe signal 36, a signature signal38 is transmitted at the boring tool 24 and received by the PDU 28. Anydeviation taken by the boring tool 24 from the preferred above-groundpath 41 is detected by the PDU 28. An appropriate course correction maybe effected either manually or automatically by the trenchlessunderground boring system 12 in response to such a deviation, as will bediscussed hereinbelow. While effecting a boring tool course change, thePDU 28 is moved along the path 41 so as to continue tracking theprogress and direction of the boring tool 24 through the ground 10. Inthis manner, cooperation between the PDU 28, the boring tool 24, and theabove-ground portion of the trenchless underground boring system 12provide for reliable and accurate navigating and tracking of anunderground boring tool 24 during excavation.

FIGS. 17 a and 17 b illustrate one embodiment of a detection methodologyemploying an antenna array 37 coupled to the PDU 28. The antenna array37 includes a left receive antenna A_(L) and a right receive antennaA_(R) which are respectively positioned on either side of a transmitantenna (not shown) situated at a mid-point between the two receiveantennas A_(L) and A_(R). The dashed line 41 shown in FIG. 17 a depictsa preferred above-ground path under which a borehole 26 is to becreated, or has been created, by a boring tool 24 equipped with acooperative target 20. At a first location L1, it can be seen that theunderground boring tool 24 is located immediately beneath the transmitantenna positioned in the center of the antenna array 37. A probe signal36 emitted by the transmit antenna at a time t₀ is received by thecooperative target 20 in the boring tool 24, which, in turn, produces asignature signal 38 that is received by the left receive antenna A_(L)and the right receive antenna A_(R) at approximately the same time, asis illustrated in the graph G₁ of FIG. 17 b.

Referring to the graph G₁ of FIG. 17 b, it is assumed that the probesignal 36 produced by the PDU 28 is transmitted at a time t₀. Becausethe two receive antennas A_(L) and A_(R) of the antenna array 37 aresubstantially equidistant relative to the cooperative target 20, thesignature signal produced by the cooperative target 20 is received bythe two antennas A_(L) and A_(R) at substantially the same time, t₁,after transmission of the probe signal at time t₀. Concurrent receptionof the signature signal by the two receive antennas A_(L) and A_(R) isdepicted in the graph G₁ of FIG. 17 b as detected signals S_(R) andS_(L), respectively, at a time t₁.

At a second location L2 along the preferred or predeterminedabove-ground path 41, it can be seen that the boring tool 24 hasdeviated in a direction left (L) of the center (C) of the predeterminedpath 41. This deviation of the boring tool 24 is detected by the PDU 28as a time delay between a time the signature signal 38 is received bythe left and right receive antennas A_(L) and A_(R), respectively. Thistime delay results from a difference in the separation distance betweenthe boring tool 24 with respect to the left and right receive antennasA_(L) and A_(R). It can be seen that the separation distance between theleft receive antenna A_(L) and the boring tool 24 is less than theseparation distance between the right receive antenna A_(R) and theboring tool 24. The boring tool deviation from the center of path 41 isreflected in the graph G₂ of FIG. 17 b as a delay between reception ofthe signature signal S_(L) by the left receive antenna A_(L) at a timet₂ and reception of the signature signal S_(R) by the right receiveantenna A_(R) at a later time t₃.

At a third location L3 further along the preferred path 41, it can beseen that the boring tool 24 has deviated to the right (R) of the center(C) of the preferred path 41. Such a deviation may result fromovercompensation when effecting a course change from a left-of-centerlocation, such as from the second location L2. The right-of-center driftof the boring tool 24 is detected by the PDU 28 as the relative timedelay between signature signal reception by the left and right receiveantennas A_(L) and A_(R), respectively. At the location L3, it can beseen that the distance between the boring tool 24 and the right receiveantenna A_(R) is less than the distance between the boring tool 24 andthe left receive antenna A_(L). Accordingly, as is indicated in thegraph G₃ of FIG. 17 b, the signature signal S_(R) is received by theright receive antenna A_(R) in advance of the signature signal S_(L)received by the left receive antenna A_(L), thereby resulting in a timedelay defined as Δ(t₃−t₂). This relative time delay may be used todetermine the magnitude of boring tool deviation from the predeterminedpath 41.

At a fourth location L4 along the predefined-above-ground path 41, itcan be seen that the boring tool 24 has been directed to the desiredcenter point location along the path 41 after having deviated to theright of the path center point at the previously discussed location L3.As is shown at location L4, the boring tool 24 is again orientatedimmediately below the center point of the antenna array 37. Thesignature signal 38 produced by the cooperative target 20 in response toa probe signal 36 emitted from the transmit antenna situated within theantenna array 37 is received substantially concurrently by the left andright receive antennas A_(L) and A_(R). The graph G₄ of FIG. 17 bdemonstrates that the boring tool 24 is once again progressing asdesired along the center line of the predetermined path 41, as evidencedby contemporaneous reception of the signature signal 38 by the left andright receive antennas A_(L) and A_(R), respectively. It is noted thatthe depth of the boring tool, d, may be determined by any of theapproaches discussed herein above. In addition, orientation of theboring tool 20 may also be detected and determined in a mannerpreviously discussed above.

FIGS. 18 a-18 b illustrate another embodiment of an antenna arrayconfiguration which may be employed in combination with the PDU 28 toaccurately track the progress of the underground boring tool 24 along anunderground path. With reference to FIG. 18 a, an antenna array 37includes four receive antennas A₁, A₂, A₃, and A₄. The antenna array 37also includes a transmit antenna (not shown) situated at a locationwithin the array 37, typically at a center location. In accordance withthis embodiment, the four receive antennas are distributed about thecircular array 37 at 0°, 90°,180°, and 270° positions, respectively. Itis to be understood that the configuration of the antenna array 37 neednot be circular as is illustrated in the figures, but may instead bearranged in any suitable geometric configuration. Also, the distributionof receive antennas about the antenna array may be different from thatillustrated in the figures.

FIG. 18 a is a depiction of the antenna array 37 having its centertransmit antenna orientated co-parallel with a predeterminedabove-ground path 41. Superimposed in FIG. 18 a is an underground boringtool 24 equipped with a cooperative target 20 depicted at threedifferent locations L1, L2, and L3 along the predetermined path 41. Atthe location L1, it can be seen that the boring tool 24 is properlyaligned co-parallel with the preferred path 41. The signature signalproduced by the cooperative target 20, in response to a probe signalproduced by the transmit antenna at a time t₀, is received atsubstantially the same time, t₄, by each of the four receive antennasA₁, A₂, A₃, and A₄. The in-phase relationship of the signature signalsS₁, S₂, S₃, and S₄ respectively received by receive antennas A₁, A₂, A₃,and A₄ is depicted in the graph G₁ of FIG. 18 b.

At a location L2, it can be seen that the boring tool 24 has deviatedright-of-center with respect to the path 41. This course deviation takenby the boring tool 24 is detected by the PDU 28 as an out-of-phasesignature signal response within the antenna array 37. Theright-of-center deviation is demonstrated in the graph G₂ of FIG. 18 bby the signature signal reception relationship associated with each ofthe four receive antennas A₁, A₂, A₃, and A₄. It can be seen that thedistance between the boring tool 24 at location L2 and the receiveantenna A₂ is less than the distance between the boring tool 24 and theother receive antennas A₁, A₃, and A₄. As is depicted in the graph G₂ ofFIG. 18 b, the signature signal S₂ is received at a time t₂ by thereceive antenna A₂ earlier than the reception times associated with theother receive antennas. By way of further example, the relativedistances between the cooperative target 24 and the receive antennas A₁and A₄ at the previous location L1 have effectively increased when theboring tool 24 deviates to the location L2, thereby increasing the delaytime of signature signal reception by receive antennas A₁ and A₄. Assuch, reception of the signature signal S₁ by antenna A₁ at a time t₇and the signature signal S₄ by receive antenna A₄ at a time t₈ isdelayed with respect to the reception of signature signal received byreceive antennas A₂ and A₃ at times t₂ and t5 respectively.

At a location L3, the graph G₃ of FIG. 18 b demonstrates that the boringtool 24 has deviated to a left-of-center position relative to the path41. The magnitude of the relative time delay within the antenna array 37indicates the magnitude of off-of-center boring tool deviations as isillustrated by the signature signal response graph of FIG. 18 b. It isnoted that the boring tool 24 may deviate beyond the periphery of theantenna array 37. Such a deviation will result in a more pronouncedreduction in the signal-to-noise ratio with respect to receive antennassituated furthest away from the boring tool location. It is understoodthat an increase in the number of receive antennas within the antennaarray 37 provides for a concomitant increase in boring tool detectionresolution. It is believed that an antenna array 37 having a diameterranging between approximately 2 feet and 5 feet is sufficient fordetecting the location of the boring tool 24 at depths of approximately10 to 15 feet or less.

In order to obtain three-dimensional data, a GPR system employingsingle-axis antenna must make several traverses over the section ofground or must use multiple antennae. The following describes theformation of two and three dimensional images in accordance with anotherembodiment of an antenna configuration used in combination with the PDU28. In FIG. 19, there is shown a section of ground 500 for which a PDU28, typically including a GPR forms an image, with a buried hazard 502located in the section of ground 500. The ground surface 504 lies in thex-y plane formed by axes x and y, while the z-axis is directedvertically into the ground 500. Generally, a single-axis antenna, suchas the one illustrated as antenna-A 506 and oriented along the z-axis,is employed to perform multiple survey passes 508. The multiple surveypasses 508 are straight line passes running parallel to each other andhave uniform spacing in the y direction. The multiple passes shown inFIG. 19 run parallel to the x-axis.

Generally, as discussed previously, a GPR system has a time measurementcapability which allows measuring of the time for a signal to travelfrom the transmitter, reflect off of a target, and return to thereceiver. After the time function capability of the GPR system providesthe operator with depth information, the radar system is moved laterallyin a horizontal direction parallel to the x-axis, thus allowing for theconstruction of a two-dimensional profile of a subsurface. By performingmultiple survey passes 508 in a parallel pattern over a particular site,a series of two-dimensional images can be accumulated to produce anestimated three-dimensional view of the site within which a buriedhazard may be located. It can be appreciated, however, that thetwo-dimensional imaging capability of a conventional antennaconfiguration 506 may result in missing a buried hazard, particularlywhen the hazard 502 is parallel to the direction of the multiple surveypasses 508 and lies in between adjacent survey passes 508.

A significant advantage of a geologic imaging antenna configuration 520of the present invention provides for true three-dimensional imaging ofa subsurface as shown in FIG. 20. A pair of antennae, antenna-A 522 andantenna-B 524, are preferably employed in an orthogonal configuration toprovide for three-dimensional imaging of a buried hazard 526. Antenna-A522 is shown as directed along a direction contained within the y-z axisand at +45° relative to the z-axis. Antenna-B 524 is also directed alonga direction contained within the y-z plane, but at −45° relative to thez-axis, in a position rotated 900 from that of antenna-A 522. It isnoted that the hyperbolic time-position data distribution typicallyobtained by use of a conventional single-axis antenna, may instead beplotted as a three-dimensional hyperbolic shape that provides width,depth, and length dimensions of a detected buried hazard 526. It isfurther noted that a buried hazard 526, such as a drainage pipeline,which runs parallel to the survey path 528 will readily be detected bythe three-dimensional imaging GPR system. Respective pairs oforthogonally oriented transmitting and receive antennae may be employedin the transmitter 54 and receiver 56 of the PDU 28 in accordance withone embodiment of the invention.

Additional features can be included on the boring tool 24. It may bedesired, under certain circumstances, to make certain measurements ofthe boring tool 24 orientation, shear stresses on the drill string 22,and the temperature of the boring tool 24, for example, in order to moreclearly understand the conditions of the boring operation. Additionally,measurement of the water pressure at the boring tool 24 may provide anindirect measurement of the depth of the boring tool 24 as previouslydescribed hereinabove.

FIG. 21 c illustrates an embodiment which allows sensors to sense theenvironment of the boring tool 410. The figure shows an active timedomain signature signal generation circuit which includes a receiveantenna 412 connected to a transmit antenna 414 through an active timedomain circuit 416. A sensor 418 is connected to the active time domaincircuit 416 via a sensor lead 420. In this embodiment, the sensor 418 isplaced at the tip of the boring tool 410 for measuring the pressure ofwater at the boring tool 410. The reading from the sensor 418 isdetected by the active time domain circuit 416 which converts thereading into a modulation signal. The modulation signal is subsequentlyused to modulate the actively generated signature signal 415. Thisprocess is described with reference to FIG. 21 d, which shows severalsignals as a function of time. The top signal 413 d represents the probesignal, I_(p), received by the receive antenna 412. The second signal,415 d, represents the actively generated signature signal I_(a), whichwould be generated if there were no modulation of the signature signal.The third trace, 416 d, shows the amplitude modulation signal, I_(m),generated by the active time domain circuit 416, and the last trace, 422d, shows the signature signal, I_(s), after amplitude modulation. Themodulated signature signal 415 is detected by the PDU 28. Subsequentdetermination of the modulation signal by the signal processor 60 in thePDU 28 provides data regarding the output from the sensor 418.

Modulation of the signature signal is not restricted to the combinationof amplitude modulation of a time domain signal as shown in theembodiment of FIG. 21. This combination was supplied for illustrativepurposes only. It is understood that other embodiments include amplitudemodulation of frequency domain signature signals, and frequencymodulation of both time and frequency domain signature signals. Inaddition, the boring tool 24 may include two or more sensors rather thanthe single sensor as illustrated in the above embodiment.

FIG. 22 a illustrates another embodiment of the invention in which aseparate active beacon is employed for transmitting information on theorientation or the environment of the boring tool 430 to the PDU 28. Inthis embodiment, shown in FIG. 22 a, the boring tool 430 includes apassive time domain signature circuit employing a single antenna 432, atime delay line 434, and an open termination 436 for reflecting theelectrical signal. The single antenna 432 is used to receive a probesignal 433 and transmit a signature/beacon signal 435. An active beaconcircuit 438 generates a beacon signal, preferably having a selectedfrequency in the range of 50 KHz to 500 MHz, which is mixed with thesignature signal generated by the termination 436 and transmitted fromthe antenna 432 as the composite signature/beacon signal 435. A mercuryswitch 440 is positioned between the active beacon circuit 438 and theantenna 432 so that the mercury switch 440 operates only on the signalfrom the active beacon circuit 438 and not on the signature signalgenerated by the termination 436.

When the boring tool 430 is oriented so that the mercury switch 440 isopen, the beacon signal circuit 438 is disconnected from the antenna432, and no signal is transmitted from the active beacon circuit 438.When the boring tool 430 is oriented so that the mercury switch 440 isclosed, the active beacon circuit 438 is connected to the antenna 432and the signal from the active beacon circuit 438 is transmitted alongwith the signature signal as the signature/beacon signal 435. The effectof the mercury switch on the signature/beacon signal 435 has beendescribed previously with respect to FIG. 21 b. The top trace 438 b, inFIG. 22 b, shows the signal, I_(b), generated by the active beaconcircuit 438 as a function of time. As the boring tool 430 rotates andmoves along an underground path, the resistance, Rm, of the mercuryswitch 440 alternates from low to high values, as shown in the centertrace 440 b. The continual opening and closing of the mercury switch 440produces a modulated signature/beacon signal 435 b, I_(m), which isreceived at the surface by the PDU 28. Only a beacon signal component,and no signature signal component, is shown in signal I_(m) 435 b. Themodulation of signal I_(m) 435 b maintains a constant phase relative toa preferred orientation of the boring tool 430. Analysis of themodulation of the beacon signal by a beacon receiver/analyzer 61 on thePDU 28 allows the operator to determine the orientation of the boringtool head.

FIG. 22 c illustrates an embodiment which allows sensors to sense theenvironment of the boring tool 450 where an active beacon is used totransmit sensor data. The figure shows an active time domain signaturesignal generation circuit including a receive antenna 452, a transmitantenna 454, and an active time domain signature signal circuit 456, allof which are connected via a time delay line 457. An active beaconcircuit 460 is also connected to the transmit antenna 454. A sensor 458is connected to the active beacon circuit 460 via a sensor lead 462. Inthis embodiment, the sensor 458 is placed near the tip of the boringtool 450 and is used to measure the pressure of water at the boring tool450. The sensor reading is detected by the active beacon circuit 460which converts the signal from the sensor 458 into a modulation signal.The modulation signal is subsequently used to modulate an active beaconsignal generated by the active beacon circuit 460.

To illustrate the generation of the signature/beacon signal 455transmitted to the PDU 28, several signals are illustrated as a functionof time in FIG. 22 d. The signal 453 d represents the probe signal,I_(p), received by the receive antenna 452. The second signal 456 drepresents the time-delayed signature signal, I_(s), generated by theactive time domain circuit 456. The third signal 460 d, I_(c),represents a combination of the time-delayed signature signal I_(s) 456d and an unmodulated signal produced by the active beacon circuit 460.The last trace, 455 d, shows a signal received at the surface, I_(m),which is a combination of the time-delayed signature signal Is 456 d anda signal produced by the active beacon circuit 460 which has beenmodulated in accordance with the reading from the sensor 458. Detectionof the modulated active beacon signal by the beacon signal detector 61in the PDU 28, followed by appropriate analysis, provides data to theuser regarding the output from the sensor 458.

In FIG. 23, there is illustrated an embodiment for using a detectionsystem to locate an underground boring tool and to characterize theintervening medium between the boring head and the PDU 28. In thisfigure, there is illustrated a trenchless underground boring system 12situated on the surface 11 of the ground 10 in an area in which theboring operation is to take place. A control unit 32 is located near thetrenchless underground boring system 12. In accordance with thisillustrative example, a boring operation is taking place under aroadway. The ground 10 is made up of several different ground types, theexamples as shown in FIG. 23 being sand (ground type (GT2)) 140, clay(GT3) 142 and native soil (GT4) 144. The road is generally described bythe portion denoted as road fill (GT1) 146. FIG. 12 illustrates a drillstring 22 in a first position 22 c, at the end of which is located aboring tool 24 c. The PDU 28 c is shown as being situated at a locationabove the boring tool 24 c. The PDU 28 c transmits a probe signal 36 cwhich propagates through the road fill and the ground.

In the case of the boring tool at location 24 c, the probe signal 36 cpropagates through the road fill 146 and the clay 142. The boring tool24 c, in response, produces a signature signal 38 c which is detectedand analyzed by the PDU 28 c. The analysis of the signature signal 38 cprovides a measure of the time-of-flight of the probe signal 36 c andthe signature signal 38 c. The time-of-flight is defined as a timeduration measured by the PDU 28 c between sending the probe signal 36 cand receiving the signature signal 38 c. The time-of-flight measureddepends on a number of factors including the depth of the boring tool 24c, the dielectric conditions of the intervening ground medium 146 and142, and any delay involved in the generation of the signature signal 38c. Knowledge of any two of these factors will yield the third from thetime-of-flight. measurement.

The depth of the boring tool 24 c can be measured independently using amechanical probe or sensing the pressure of the water at the boring tool24 c using a sensor 130 located in the boring tool head 24 c asdiscussed hereinabove. For the latter measurement, the boring operationis halted, and the water pressure measured. Since the height of thewater column in the drill string 22 above the ground is known, the depthof the boring tool 24 c can be calculated using known techniques. For anembodiment of the invention which uses a microwave probe signal, ageneral relationship for calculating the depth or dielectric constantfrom the time of flight measurement is given by Equation [8] discussedpreviously hereinabove.

For the case where the boring tool is located at position 24 c as shownin FIG. 23, and with the assumption that the road fill has a negligiblethickness relative to the thickness of clay, the relationship ofEquation [8] simplifies to: $\begin{matrix}{{TE} = {{{TF} - {TD}} = {\sum\frac{d_{3}\sqrt{ɛ_{3}}}{c}}}} & \lbrack 9\rbrack\end{matrix}$where, the subscript “3” refers to GT3. Direct measurement of thetime-of-flight, TF, and the depth of the boring tool 24 c, d₃, alongwith the knowledge of any time delay, TD, will yield the averagedielectric constant, M₃, of GT3. This characteristic can be denoted asGC3.

Returning to FIG. 23, there is illustrated an embodiment in which theboring tool 24 has been moved from its first location 24 c to anotherposition 24 d. The drill string 22 d (shown in dashed lines) has beenextended from its previous configuration 22 c by the addition of extradrill string members in a manner as described previously hereinabove.The PDU 28 has been relocated from its previous position 28 c to a newposition 28 d (shown in dashed lines) in order to be close to the boringtool 24 d. The parameter GC4, which represents the ground characteristicof the native soil GT4, can be obtained by performing time-of-flightmeasurements as previously described using the probe signal 36 d andsignature signal 38 d. Likewise, ground characteristic GC2 can beobtained from time-of-flight measurements made at the point indicated bythe letter “e”. The continuous derivation of the ground characteristicsas the boring tool 24 d travels through the ground results in theproduction of a ground characteristic profile which may be recorded bythe control unit 32. The characteristics of the intervening groundmedium between the PDU 28 and the cooperative target 20 may bedetermined in manner described herein and in U.S. Pat. No. 5,553,407,which is assigned to the assignee of the instant application, thecontents of which are incorporated herein by reference.

It may be advantageous to make a precise recording of the undergroundpath traveled by the boring tool 24. For example, it may be desirable tomake a precise record of where utilities have been buried in order toproperly plan future excavations or utility burial and to avoidunintentional disruption of such utilities. Borehole mapping can beperformed manually by relating the boring tool position data collectedby the PDU 28 to a base reference point, or may be performedelectronically using a Geographic Recording System (GRS) 150 showngenerally as a component of the control unit 32 in FIG. 24. In oneembodiment, a Geographic Recording System (GRS) 150 communicates with acentral processor 152 of the control unit 32, relaying the preciselocation of the PDU 28. Since the control unit 32 also receivesinformation regarding the position of the boring tool 24 relative to thePDU 28, the precise location of the boring tool 24 can be calculated andstored in a route recording database 154.

In accordance with another embodiment, the geographic position dataassociated with a predetermined underground boring route is acquiredprior to the boring operation. The predetermined route is calculatedfrom a survey performed prior to the boring operation. The prior surveyincludes GPR sensing and geophysical data in order to estimate the typeof ground through which the boring operation will take place, and todetermine whether any other utilities or buried hazards are located on aproposed boring pathway. The result of the pre-bore survey is apredetermined route data set which is stored in a planned route database156. The predetermined route data set is uploaded from the planned routedatabase 156 into the control unit 32 during the boring operation toprovide autopilot-like directional control of the boring tool 24 as itcuts its underground path. In yet another embodiment, the position dataacquired by the GRS 150 is preferably communicated to a route mappingdatabase 158 which adds the boring pathway data to an existing databasewhile the boring operation takes place. The route mapping database 158covers a given boring site, such as a grid of city streets or a golfcourse under which various utility, communication, plumbing and otherconduits may be buried. The data stored in the route mapping database158 may be subsequently used to produce a survey map that accuratelyspecifies the location and depth of various utility conduits buried in aspecific site. The data stored in the route mapping database 158 alsoincludes information on boring conditions, ground characteristics, andprior boring operation productivity, so that reference may be made bythe operator to all prior boring operational data associated with aspecific site.

An important feature of the novel system for locating the boring tool 24concerns the acquisition and use of geophysical data along the boringpath. A logically separate Geophysical Data Acquisition Unit 160 (GDAU),which may or may not be physically separate from the PDU 28, may providefor independent geophysical surveying and analysis. The GDAU 160preferably includes a number of geophysical instruments which provide aphysical characterization of the geology for a particular boring site. Aseismic mapping module 162 includes an electronic device consisting ofmultiple geophysical pressure sensors. A network of these sensors isarranged in a specific orientation with respect to the trenchlessunderground boring system 12, with each sensor being situated so as tomake direct contact with the ground. The network of sensors measuresground pressure waves produced by the boring tool 24 or some otheracoustic source. Analysis of ground pressure waves received by thenetwork of sensors provides a basis for determining the physicalcharacteristics of the subsurface at the boring site and also forlocating the boring tool 24. These data are processed by the GDAU 160prior to sending analyzed data to the central processor 152.

A point load tester 164 may be employed to determine the geophysicalcharacteristics of the subsurface at the boring site. The point loadtester 164 employs a plurality of conical bits for the loading pointswhich, in turn, are brought into contact with the ground to test thedegree to which a particular subsurface can resist a calibrated level ofloading. The data acquired by the point load tester 164 provideinformation corresponding to the geophysical mechanics of the soil undertest. These data may also be transmitted to the GDAU 160.

The GDAU 160 may also include a Schmidt hammer 166 which is ageophysical instrument that measures the rebound hardnesscharacteristics of a sampled subsurface geology. Other geophysicalinstruments may also be employed to measure the relative energyabsorption characteristics of a rock mass, abrasivity, rock volume, rockquality, and other physical characteristics that together provideinformation regarding the relative difficulty associated with boringthrough a given geology. The data acquired by the Schmidt hammer 166 arealso stored in the GDAU 160.

In the embodiment illustrated in FIG. 24, a Global Positioning System(GPS) 170 is employed to provide position data for the GRS 150. Inaccordance with a U.S. Government project to deploy twenty-fourcommunication satellites in three sets of orbits, termed the GlobalPositioning System (GPS), various signals transmitted from one or moreGPS satellites may be used indirectly for purposes of determiningpositional displacement of a boring tool 24 relative to one or moreknown reference locations. It is generally understood that the U.S.Government GPS satellite system provides for a reserved, or protected,band and a civilian band. Generally, the protected band provides forhigh-precision positioning to a classified accuracy. The protected band,however, is generally reserved exclusively for military and othergovernment purposes, and is modulated in such a manner as to render itvirtually useless for civilian applications. The civilian band ismodulated so as to significantly reduce the accuracy available,typically to the range of one hundred to three hundred feet.

The civilian GPS band, however, can be used indirectly in relativelyhigh-accuracy applications by using one or more GPS signals incombination with one or more ground-based reference signal sources. Byemploying various known signal processing techniques, generally referredto as differential global positioning system (DGPS) signal processingtechniques, positional accuracies on the order of centimeters are nowachievable. As shown in FIG. 24, the GRS 150 uses the signal produced byat least one GPS satellite 172 in cooperation with signals produced byat least two base transponders 174, although the use of one basetransponder 174 may be satisfactory in some applications. Various knownmethods for exploiting DGPS signals using one or more base transponders174 together with a GPS satellite 172 signal and a mobile GPS receiver176 coupled to the control unit 32 may be employed to accurately resolvethe boring tool 24 movement relative to the base transponder 174reference locations using a GPS satellite signal source.

In another embodiment, a ground-based positioning system may be employedusing a range radar system 180. The range radar system 180 includes aplurality of base radio frequency (RF) transponders 182 and a mobiletransponder 184 mounted on the PDU 28. The base transponders 182 emit RFsignals which are received by the mobile transponder 184. The mobiletransponder 184 includes a computer which calculates the range of themobile transponder 184 relative to each of the base transponders 182through various known radar techniques, and then calculates its positionrelative to all base transponders 182. The position data set gathered bythe range radar system 180 is transmitted to the GRS 150 for storing inroute recording database 154 or the route mapping system 158.

In yet another embodiment, an ultrasonic positioning system 190 may beemployed together with base transponders 192 and a mobile transponder194 coupled to the PDU 28. The base transponder 192 emits signals havinga known clock timebase which are received by the mobile transponder 194.The mobile transponder 194 includes a computer which calculates therange of the mobile transponder 194 relative to each of the basetransponders 192 by referencing the clock speed of the source ultrasonicwaves. The computer of the mobile transponder 194 also calculates theposition of the mobile transponder 194 relative to all of the basetransponders 192. It is to be understood that various other knownground-based and satellite-based positioning systems and techniques maybe employed to accurately determine the path of the boring tool 24 alongan underground path.

FIG. 25 illustrates an underground boring tool 24 performing a boringoperation along an underground path at a boring site. An importantadvantage of the novel geographic positioning unit 150, generallyillustrated in FIG. 25, concerns the ability to accurately navigate theboring tool 24 along a predetermined boring route and to accurately mapthe underground boring path in a route mapping database 158 coupled tothe control unit 32. It may be desirable to perform an initial survey ofthe proposed boring site prior to commencement of the boring operationfor the purpose of accurately determining a boring route which avoidsdifficulties, such as previously buried utilities or other obstacles,including rocks, as is discussed hereinbelow.

As the boring tool 24 progresses along the predetermined boring route,actual positioning data are collected by the geographic recording system150 and stored in the route mapping database 158. Any intentionaldeviation from the predetermined route stored in the planned pathdatabase 156 is accurately recorded in the route mapping database 158.Unintentional deviations are corrected so as to maintain the boring tool24 along the predetermined underground path. Upon completion of a boringoperation, the data stored in the route mapping database 158 may bedownloaded to a personal computer (not shown) to construct an “as is”underground map of the boring site. Accordingly, an accurate map ofutility or other conduits installed along the boring route may beconstructed from the route mapping data and subsequently referenced bythose desiring to gain access to, or avoid, such buried conduits.

Still referring to FIG. 25, accurate mapping of the boring site may beaccomplished using a global positioning system 170, range radar system180 or ultrasonic positioning system 190 as discussed previously withrespect to FIG. 24. A mapping system having a GPS system 170 includesfirst and second base transponders 600 and 602 together with one or moreGPS signals 606 and 608 received from GPS satellites 172. A mobiletransponder 610, coupled to the control unit 32, is provided forreceiving the GPS satellite signal 606 and base transponder signals 612and 614 respectively transmitted from the transponders 600 and 602 inorder to locate the position of the control unit 32. As previouslydiscussed, a modified form of differential GPS positioning techniquesmay be employed to enhance positioning accuracy to the centimeter range.A second mobile transponder 616, coupled to the PDU 28, is provided forreceiving the GPS satellite signal 608 and base transponder signals 618and 620 respectively transmitted from the transponders 600 and 602 inorder to locate the position of the PDU 28.

In another embodiment, a ground-based range radar system 180 includesthree base transponders 600, 602, and 604 and mobile transponders 610and 616 coupled to the control unit 32 and PDU 28, respectively. It isnoted that a third ground-based transponder 604 may be provided as abackup transponder for a system employing GPS satellite signals 606 and608 in cases where GPS satellite signal 606 and 608 transmission istemporarily terminated, either purposefully or unintentionally. Positiondata for the control unit 32 are processed and stored by the GRS 150using the three reference signals 612, 614, and 622 received from theground-based transponders 600, 602, and 604, respectively. Position datafor the PDU 28, obtained using the three reference signals 618, 620, and624 received respectively from the ground-based transponders 600, 602,and 604, are processed and stored by the local position locator 616coupled to the PDU 28 and then sent to the control unit 32 via a datatransmission link 34. An embodiment employing an ultrasonic positioningsystem 190 would similarly employ three base transponders 600, 602, and604, together with mobile transponders 610 and 616 coupled to thecontrol unit 32 and PDU 28, respectively.

Referring now to FIG. 26, there is illustrated in flowchart formgeneralized steps associated with the pre-bore survey process forobtaining a pre-bore site map and determining the optimum route for theboring operation prior to commencing the boring operation. In brief, apre-bore survey permits examination of the ground through which theboring operation will take place and a determination of an optimumroute, an estimate of the productivity, and an estimate of the cost ofthe entire boring operation.

Initially, as shown in FIG. 26, a number of ground-based transpondersare positioned at appropriate locations around the boring site at step300. The control unit 32 and the PDU 28 are then situated at locationsL0 and L1 respectively at step 302. The geographical recording system150 is then initialized and calibrated at step 304 in order to locatethe initial positions of the control unit 32 and PDU 28. Aftersuccessful initialization and calibration, the PDU 28 is moved along aproposed boring route, during which PDU data and geographical locationdata are acquired at steps 306 and 308, respectively. The data gatheredby the PDU 28 are preferably analyzed at steps 306 and 308. Theacquisition of data continues at step 312 until the expected end of theproposed boring route is reached, at which point data accumulation ishalted, as indicated at step 314.

The acquired data are then downloaded to the control unit 32, which maybe a personal computer, at step 316. The control unit 32, at step 318,then calculates an optimum pre-determined path for the boring operation,and does so as to avoid obstacles and other structures. If it is judgedthat the pre-determined route is satisfactory, as is tested at step 320,the route is then loaded into the planned path database 156 at step 322,and the pre-bore survey process is halted at step 324. If, however, itis determined that the planned route is unsatisfactory, as is tested atstep 320 because, for example, the survey revealed that the boring tool24 would hit a rock obstacle or that there were buried utilities whichcould be damaged during a subsequent boring operation, then the PDU 28can be repositioned, at step 326, at the beginning of the survey routeand a new route surveyed by repeating steps 304-318. After asatisfactory route has been established, the pre-bore survey process ishalted at step 324.

In another embodiment, the pre-bore survey process includes thecollection of geological data along the survey path, concurrently withposition location and PDU data collection. This collection activity isillustrated in FIG. 26 which shows an initialization and calibrationstep 328 for the geophysical data acquisition unit 160 (GDAU) takingplace concurrently with the initialization and calibration of thegeographical recording system 150. The GDAU 160 gathers geological dataat step 330 at the same time as the PDU 28 and position data are beingacquired in steps 306 and 308, respectively. The inclusion of geologicaldata gathering provides for a more complete characterization of theground medium in the proposed boring pathway, thus allowing for moreaccurate productivity and cost estimates to be made for the boringoperation.

In a third embodiment, the survey data are compared with previouslyacquired data stored in the route mapping database 158 in order toprovide estimates of the boring operation productivity and cost. In thisembodiment, historical data from the route mapping database are loadedinto the control processor 152 at step 332 after the survey data havebeen downloaded to the control unit 32 in step 316. The data downloadedfrom the route mapping database 158 include records of prior surveys andboring operations, such as GPR and geological characterizationmeasurements and associated productivity data. The pre-planned route iscalculated at step 334 in a manner similar to the calculation of theroute indicated at step 318. By correlating the current groundcharacterization, resulting from the PDU 28 and GDAU 160 data, withprior characterization measurements and making reference to associatedprior productivity results, it is possible to estimate, at step 336,productivity data for the planned boring operation. Using the estimatedproduction data of step 336, it is then possible to produce a costestimate of the boring process at step 338. In the following step 320, adetermination is made regarding whether or not the pre-planned route issatisfactory. This determination can be made using not only thesubsurface features as in the first embodiment, but can be made usingother criteria, such as the estimated duration of the boring process orthe estimated cost.

Referring now to FIG. 27, there is illustrated a system block diagram ofa control unit 32, its various components, and the functionalrelationship between the control unit 32 and various other elements ofthe trenchless underground boring system 12. The control unit 32includes a central processor 152 which accepts input data from thegeographic recording system 150, the PDU 28, and the GDAU 160. Thecentral processor 152 calculates the position of the boring tool 24 fromthe input data. The control processor 152 records the path taken by theboring tool 24 in the route recording database 154 and/or adds it to theexisting data in the route mapping database 158.

In an alternative embodiment, the central processor 152 also receivesinput data from the sensors 230 located at the boring tool 24 throughthe sensor input processor 232. In another embodiment, the centralprocessor 152 loads data corresponding to a predetermined path from theplanned path database 156 and compares the measured boring tool positionwith the planned position. The position of the boring tool 24 iscalculated by the central processor 152 from data supplied by the PDUinput processor 234 which accepts the data received from the PDU 28. Inan alternative embodiment, the central processor 152 also employs dataon the position of the PDU 28, supplied by the Geographic RecordingSystem 150, in order to produce a more accurate estimate of the boringtool location.

Corrections in the path of the boring tool 24 during a boring operationcan be calculated and implemented to return the boring tool 24 to apredetermined position or path. The central processor 152 controlsvarious aspects of the boring tool operation by use of a trenchlessground boring system control (GBSC) 236. The GBSC 236 sends controlsignals to boring control units which control the movement of the boringtool 24. These boring control units include the rotation control 238,which controls the rotating motor 19 for rotating the drill string 22,the thrust/pullback control 242, which controls the thrust/pullback pump20 used to drive the drill string 22 longitudinally into the borehole,and the direction control 246, which controls the direction activatormechanism 248 which steers the boring tool 24 in a desired direction.The PDU input processor 234 may also identify buried features, such asutilities, from data produced by the PDU 28. The central processor 152calculates a path for the boring tool 24 which avoids any possibility ofa collision with, and subsequent damage to, such buried features.

In FIGS. 28 and 29, there are illustrated flow charts for generalizedprocess and decision steps associated with boring a trenchless holethrough the ground. Initially, as shown in FIG. 28 and at step 350, anumber of ground-based transponders are positioned at appropriatelocations around a boring site. The trenchless underground boring system12 is then positioned at the appropriate initial location, as indicatedat step 352, and the transponders and geographic recording system areinitialized and calibrated, at step 354, prior to the commencement ofboring, at step 356. After boring has started, the PDU 28 probes theground at step 358 and then receives and analyzes the signature signalat step 360. Independent of, and occurring concurrently with, theprobing and receiving steps 358 and 360, the GRS receives position dataat step 362 and determines the position of the PDU 28 at step 364. Aftersteps 362 and 364 have been completed, the central processor 152 thendetermines the position of the boring tool 24 at step 366.

The central processor 152 then compares the measured position of theboring tool 24 with the expected position, at step 368, as given in theplanned path database 156 and calculates whether or not a correction isrequired to the boring tool direction, at step 370, and provides acorrection at step 372, if necessary. The trenchless underground boringsystem 12 continues to bore through the ground at step 374 until theboring operation is completed as indicated at steps 376 and 378. If,however, the boring operation is not complete, the central processor 152decides, at step 380, whether or not the PDU 28 should be moved in orderto improve the image of the boring tool 24. The PDU 28 is then moved ifnecessary at step 382 and the probing and GRS data reception steps 358and 362 recommence. The operation is halted after the boring tool 24 hasreached a final destination.

In an alternative embodiment, shown in dashed lines in FIGS. 28 and 29,the central processor 152 records, at step 384, the calculated positionof the boring tool 24 in the route mapping database 158 and/or the routerecording database 154, after determining the position of the boringtool at step 366. In another embodiment, the steps of comparing (step368) the position of the boring tool 24 with a pre-planned position andgenerating any necessary corrections (steps 370 and 372) are omitted asis illustrated by the dashed line 386.

It will, of course, be understood that various modifications andadditions can be made to the preferred embodiments discussed hereinabovewithout departing from the scope or spirit of the present invention.Accordingly, the scope of the present invention should not be limited bythe particular embodiments described above, but should be defined onlyby the claims set forth below and equivalents thereof.

1. A method, for use in horizontal directional drilling, ofcommunicating information between an above-ground locator and asub-surface beacon, the method comprising: transmitting a locator signalfrom the locator; detecting the locator signal at the beacon;determining a position of the beacon relative to the locator in responseto the locator signal detected at the beacon and the orientation of thebeacon; transmitting from the beacon a beacon signal containinginformation relating to the position of the beacon; and receiving at thelocator information related to the position of the beacon.
 2. The methodof claim 1, comprising modulating the beacon signal with an informationsignal.
 3. The method of claim 2, comprising extracting the informationsignal from the beacon signal at the locator.
 4. The method of claim 2,wherein the information signal is an operational parameter associatedwith the beacon assembly.
 5. The method of claim 2, wherein theinformation signal is an orientation signal indicative of an orientationof the beacon assembly.
 6. The method of claim 1, comprising: modulatingthe beacon signal with data relating to an orientation of the beaconassembly; and processing the modulated beacon signal at the locator toextract the orientation data.
 7. The method of claim 1, comprisingdisplaying the orientation data.
 8. The method of claim 1, comprisingproducing three-dimensional image data of the beacon assembly within thesub-surface.
 9. The method of claim 1, wherein each of the locatorsignal and the beacon signal comprises an electromagnetic signal.
 10. Asystem for use in horizontal directional drilling, comprising: anabove-ground locator, comprising: a transmitter arranged to transmit alocator signal in a substantially vertical direction into a subsurface;a receiver arrangement comprising at least one receive antenna; and aprocessor; and a beacon assembly configured for sub-surface deployment,comprising: a beacon receiver arrangement comprising an antennaarrangement adapted to detect the locator signal transmitted from theabove-ground locator; an orientation sensor adapted to sense anorientation of the beacon assembly; circuitry coupled to the orientationsensor and beacon receiver arrangement, the circuitry configured toproduce a beacon signal used to determine a position of the beaconassembly relative to the above-ground locator in response to the locatorsignal received by the beacon receiver arrangement and the orientationof the beacon assembly; and a beacon transmitter configured to transmitthe beacon signal to an above-ground location; whereby the locatorreceiver arrangement is configured to receive the beacon signal and thelocator processor is configured to determine the position of the beaconassembly with respect to the locator using the received beacon signal.11. The system of claim 10, wherein the locator receiver arrangementcomprises a signal antenna.
 12. The system of claim 10, wherein thelocator receiver arrangement comprises first, second, and third antennasoriented orthogonal to each other.
 13. The system of claim 10, whereinthe beacon assembly comprises first, second, and third antennas orientedorthogonal to each other.
 14. The system of claim 10, wherein theorientation sensor comprises an accelerometer and the orientation of thebeacon assembly comprises a pitch of the beacon assembly.
 15. The systemof claim 10, wherein the orientation sensor comprises an accelerometerand the orientation of the beacon assembly comprises a roll of thebeacon assembly.
 16. The system of claim 10, wherein: the beacontransmitter is configured to transmit the beacon signal containingbeacon assembly orientation information; and the locator processor isconfigured to determine the orientation of the beacon assembly inresponse to the orientation information contained in the beacon signaland produce orientation data in a form suitable for display.
 17. Thesystem of claim 10, wherein the locator comprises a display and whereinthe locator processor is configured to produce data for producing animage showing the position of the beacon assembly on the display. 18.The system of claim 10, wherein the locator comprises a display andwherein the locator processor is configured to produce data forproducing an image showing a direction of the beacon assembly on thedisplay.
 19. The system of claim 10, wherein the locator processor isconfigured to calculate a depth of the beacon assembly in response tothe beacon signal received by the locator receiver arrangement.
 20. Thesystem of claim 10, wherein each of the locator signal and the beaconsignal comprises an electromagnetic signal.